Most of the men who write me wanting to improve their erectile dysfunction or low testosterone are significantly overweight. The truth is that they don't need to be told that their spare tire is an issue - they're experiencing the symptoms first hand!
Many of them have struggled all their lives with weight issues and many of you reading this are in the same situation. Rest assured, though, that losing weight does not have to be the laborious, painful experience that so many make it to be. The key is to understand your body, your hormones and a few tips and tricks that the research has uncovered. For example, did you know that one study showed that if you lose weight by diet along you will lose muscle?  However, if you add exercise to your diet, you will lose primarily fat and maintain your muscle. Again, many people, even in the health community, are unaware of this fact.
Now, before I move on and discuss the best way to lose weight, I want to mention that more than anything else it is perhaps most critical that you not lose weight too rapidly. You will have trainers, your conscience and maybe a few friends and family pushing you to drop those pounds quickly. Well, don't listen! Losing weight rapidly destroys your testosterone (and leutinizing hormone) like nothing else. A recent Finnish study found that wrestlers who dropped weight very quickly decreased testosterone levels by over 60%!  Ouch! (See the bottom for more details.)
So my advice is relax and lose weight slowly, steadily and peacefully with dignity and grace. No need for television theatrics. No need for family and friends or even yourself to suffer immeasurably. Losing weight is not a monumental decision - it's just a part of life for all of us.
If you need motivation, read here about the Many Dangers of Being Overweight. And here's some scientific findings that will really help:
1) Muscle. If you have lost a lot of muscle over the years, you will find it easy to put on weight. Remember that "Muscle Burns Fat". If you can trade five pounds of fat for five pounds of muscle, you will burn 200 more calories per day! This translates to at least 21 pounds per year. A lot of people wonder why they gain weight so easily and it's because they've let their muscle mass slowly decline, which leads to a nasty metabolic downward spiral.
2) Fructose. Watch the fructose! Read here about How Fructose Sabotages Weight Loss.
3) Green Tea Extract. One supplement will help you lose weight is Green Tea Extract. Animal experiments have shown that mice (with an obesity gene) experienced less weight gain with Green Tea Extract and had lower cholesterol and triglyceride levels as well.  Another 2008 study showed that rats had less weight gain while on Green Tea Extract.  Previous studies had shown that Green Tea does this through thermogenesis , basically increasing one's metabolism and fat burning, but these animal studies showed additional mechanisms. But additional research has shown that Green Tea inhibits fat-digesting enzymes  and helps you eliminate (poop) a higher percentage of fat after ingesting a high fat meal. 
4) Exercise. I don't need to tell you that exercise burns calories.  I have known a number of people who could not lose weight until they started exercising and exercise makes you less insulin resistant. Regardless of the reason, you may need to couple exercise with your weight loss program.
5) Insulin Resistance. Read my link on Metabolic Syndrome. One of the hallmarks of Metabolic Syndrome is insulin resistance and scientists have just discovered that high insulin levels keep your body from breaking down fat. An insulin resitant individual will have high insulin levels and high glucose concentrations, a toxic combination that causes damage in different ways. The high insulin levels are deadly because the block the breakdown of fats by adrenaline and lipase.  In other words, insulin resistance makes it MUCH easier for to retain fat, so it is critical that you put in your gym or road time.
6) Low Glycemic Index. One recent 12-week study found that eating a low glycemic diet produced greater weight loss than those on a high glycemic diet.  Low glycemic foods are those that do not spike your blood sugar as much, such as whole grains and most fruits and vegetables. NOTE: It should be noted, though, that not all studies have found these results. Glycemic Load is, by the way, a much more important concept actually than the Glycemic Index.
7) Exercise to Increase Basal Metabolism. As we age our metabolism slows down, largely due to the loss of lean muscle mass. "Muscle burns fat" is a saying that is largely true. Exercise is one of the best ways to ramp up your metabolism and burn off calories. Plus, most forms of exercise will also build some muscle for you as well. The book, The GI Mediterranean Diet - an excellent read by the way - talks about how if you exercise in the morning for even 20 minutes, you increase your metabolism for the next 10-12 hours!  Yet another study found that weight training (8 exercises, 8 sets, and 6 repetitions) increased baseline energy expenditure for at least 72 hours afterward!  If you exercise in the evening, you only get this boost in metabolism for four or five hours until you go to bed. Of course, the best thing to do is exercise for a half hour in the morning and then a half hour in the evening and you'll be burning calories every waking hour.
8) Squats. Do strength and weight training on your large muscle groups, especially the quads (thighs) and gluts (butt). Researchers at Syracuse University found that the body takes more energy afterwards to repair and rebuild and so more calories are burned off. 
9) Avoid Late Night Snacks. You still here "calories in, calories out" is all that matters in weight gain and weight loss. That belief is slowly getting shattered of lateResearchers recently found that late night snacks affect metabolism negatively and lead to greater weight gain.  Mice fed late at night gained over twice as much weight as mice fed during regular feeding hours.
10) Sitting and Lipase. Scientists have found that sitting does something particularly nasty: it shuts off the activity of an all-important weight loss enzyme called lipase. It does this by severely limiting the circulation of lipase and lipase is critical because it allows the body to process fat and cholesterol. Just the simple act of standing allows for a much, much better dispersion of lipase throughout your tissues. If you are sedentary and haven't been able to lose weight, now you know one of the reasons why. It's not just that you have been burning enough calories, it's that you have been limiting your body's natural chemical processing factories. So, the old expression "Get off your a#$%!" might actually save one's life!
11) High-Intensity Training. If you can take the pain, High-Intensity Training, a close relative of Interval Training, has also been shown to help you lose weight the fastest. Intensity is a technical term that basically refers to the amount of effort in a certain amount of time or the amount of "sweat and burn" the exercise induces. High-Intensity Training actually boosts your metabolism so that you "burn fat" for more hours than other training methods.  Researchers put (female) participants on a repeated cyle of "sprinting" on a stationary bike for 12 seconds and then pedaling much more slowly for 12 seconds and so on for twenty minutes three times per week.  This very minimal amount of exercise significantly reduced body fat and controlled blood sugar metabolism.
12) GoWear/Sensewear/Bodybugg. This is the device used on Biggest Loser and with good reason: many guys may benefit from the use of this armband which uses a complex set of readings on acceleration, skin temperature/response and heat to determine how many calories you have burned. It is supposedly much more accurate than standard heart monitors and considering that a lot of us are motivated by numbers, myself included, this offers a considerable level of detail. You get numbers and charts galore as it is used in conjunction with a web site that lets you analyze your progress.
13) Sleep. Sleep is a key to keeping off the pounds. It is so important that I have created a separate link How Lack of Sleep Sabotages Weight Loss.
14) Low Fat Diet. The beauty of a Low Fat (or Ornish) Diet is that it solves your hunger problem during dieting, because you can eat as much as you want. Think how much you can eat if you are eating nothing but (whole) grains, beans, legumes, vegetables, fruit and egg whites? You can fill your stomach up many times per day and still lose weight because there is no high-calorie fat in your gut. Fat is 9 calories per gram, whereas carbohydrates and protein are only 4 calories per gram. The typical American eats a diet around 35-40% fat and so the typical American end up eating too many calories.
In fact, the astonishing thing is that you will get full before you actually get all the calories. In other words, you'll actually lose weight on a Low Fat (or Ornish) Diet. Dr. Ornish performed a study and found that people who followed his diet lost an average of 22 pounds in their first year.  Imagine almost effortlessly losing weight while you walk around full and satisfied. In other words, forget dieting -just eat the right way!
15) Vinegar: Vinegar has just been found in an extremely promising animal study to lower visceral fat. Visceral fat is that annoying and dangerous fat that most guys get around their abdomen, i.e. a paunch/spare tire/gut. Lab animals who consumed vinegar had 10% lower fat. Please about How Vinegar Can Help You Maintain (or Lower) Your Weight.
16) Appetite. One of keys to weight control is controlling your appetite. Packaged and processed food, in particular, is packed with sugar, salt and fat and compel you to eat more than you need. On the other side, there are actually many healthy foods that are particularly satisfying. Read this link about Foods that Help Control Appetite.
17) Stress. Watch your stress levels. Researchers recently found that on average chronic stress led to weight gain, due to "eating in the absence of hunger". 
18) Whey. Research, some of it quite recent, has shown that whey will very likely help you lose weight. Whey, a common bodybuilding protein supplement, does this through satiety of the appetite , control of glucose and insulin post-meal  and muscle building (and muscle burns more calories). Even more interesting is the fact that whey appears to actually raise the body's Resting Energy Expenditure, thus making your system a fat-burning inferno.  In other words, whey seems to help you burn off those calories in almost every way possible. CAUTION: Almost all store bought wheys that I have seen are loaded with dangerous excitotoxins and I document that here in my link called No Whey!. However, there is an alternative, undenatured or biologically active whey, which I document in my link called Yes Whey if you want more information.
NOTE 1: Chromium, in spite of what your health food store may have told you, has been shown to be ineffective for weight loss. However, it does seem to help your body manage blood sugar. So there is some justification for taking chromium but not as miracle weight loss supplement.
NOTE 2: For those of you who are Mediterranean Diet aficionados, you'll be glad to know that a recent meta-analysis found that most studies showed a decreased risk for becoming obese for those who followed a Mediterranean Diet.  Again, the Mediterranean Diet is relatively high fat (~30%), making it easier to put on weight. However, it does convey a low glycemic load, which may contribute to its ability to keep off weight gain. And one could also speculate that perhaps because it is fulfilling and satisfying, it leaves its practioners eating somewhat less.
NOTE 3: You can't trust the calorie labelling on food items.  The problem is that our bodies digest food and don't burn it, which is where the whole calorie calculations are based on. This is important because protein and fiber actually take significant energy to digest whereas cheap carbs do not. So if one consumes a meal with a lot of protein and fiber, the amount of calories actually stored by the body may be as much as 25% less than that with sugars and other less healthy foods. Yes, this is yet another reason to avoid the bad stuff.
1) Metabolism, 2001, 50(7):795 - 800
3) Amer J of Clin Nutrition,Jun 1984,39(6):930-6
6) Obesity Reviews, Nov 2008, 9(6):582-593
7) Amer Soc for Nutrition J Nutr, Feb 2008, 138:323-331
8) Amer Soc for Nutrition J. Nutr, Nov 2008, 138:2156-2163
9) Int J Obes Relat Metab Disord, 2000 Feb, 24(2):252-8; J Med Food, 2006, 9(4):451-8
10) J Nutr Biochem, 2000 Jan, 11(1):45-51
11) Eur J Clin Nutr, Nov 2006, 60(11):1330-1336
12) Nutr J, 2008, submitted
13) West Afr J Med, Apr-Jun 1990, 9(2):108-115
14) Karila, TA., et al. Rapid Weight Loss Decreases Serum Testosterone. (2008) Int. Journal of Sports Medicine. May 30
16) Eur J of Clin Nutr, 2008, 62:145-149
17) Brit J of Nutr, 2001, 85:271-287, A perspective on food energy standards for nutrition labelling, Geoffrey Livesey
18) The GI Mediterranean Diet, Dr. Fedon Alexander Lindberg, p. 89-90.
19) Men's Health, Sep 2009, p. 38.
20) J of Strength and Conditioning Res, Sep 2008, 22(5):1602-1609, "Resting Energy Expenditure and Delayed-Onset Muscle Soreness After Full-Body Resistance Training With an Eccentric Concentration", Hackney, Kyle J
21) Obesity, "Circadian Timing of Food Intake Contributes to Weight Gain", Arble, et. al.
22) Intl J of Obesity, 2008, 32:684-691, "The effects of high-intensity intermittent exercise training on fat loss and fasting insulin levels of young women"
23) International Journal of Obesity, 2001, 25:332-339, "Impact of high-intensity exercise on energy expenditure, lipid oxidation and body fatness"
25) The Amer J of Med, 95(2):131-140, "Effect of an energy-restrictive diet, with or without exercise, on lean tissue mass, resting metabolic rate, cardiovascular risk factors, and bone in overweight postmenopausal women"
26) Obesity, 2008, 17(1):72 77, "Acute Stress-related Changes in Eating in the Absence of Hunger"
27) British Journal of Nutrition, 2003 Feb, 89(2):239-48, "Casein and whey exert different effects on plasma amino acid profiles, gastrointestinal hormone secretion and appetite"
28) FASEB Journal, "The Effect of Whey Protein on Post-Meal Blood Glucose and Insulin"
29) Med Sci Sports Exercise, 2009 Dec 4, [Epub ahead of print], "Timing Protein Intake Increases Energy Expenditure 24 Hours Post-Resistance Training"