Just this week (6/22/2009) one of the most important testosterone-related discoveries was uncovered, although it was one of those things many of us were saying, "I always wondered!" It was found that glucose lowered testosterone. As you may know, a glucose drink is often given before a fasting blood sugar test and researchers, commissioned by the Endocrine Society, administered the standard 75 gram glucose test and monitored testosterone after ingestion of the drink The results surprised everyone: testosterone was whacked for hours after the glucose drink.
The extent of the hormonal effect was shocking. First of all, testosterone was diminished by up to 25% regardless of whether the men were diabetic, prediabetic or healthy. In other words, glucose greatly decreased testosterone in everyone. Secondly, 98.6% of the study participants had significantly lower testosterone two hours after taking the glucose! Finally, 15% of the subjects were actually driven into the hypogonadal range, i.e. their testosterone would have been flagged as clinically low.
Many bodybuilders drink a high glycemic drink with protein after a weight lifting workout in order to produce an insulin spike to push amino acids into their cells for muscle building. In light of the above, this may not be such an enlightened practice.
NOTE: There's another reason that you may not want to drink that sugary drink: Fructose Sabotages Weight Loss. Another interesting fact is that this same effect probably holds true with females as well. Scientists put a group of females on low and high glycemic load diets with a washout period in between. What they found was that females produced on average 22% less testosterone on the high glycemic diet versus the low. 
Okay, so testosterone is diminished significantly by glucose - but what about the table sugar and corn syrup that are in almost everything that we eat? The answer is that these will almost for sure affect testosterone in a similar way because table sugar is one half glucose, one half fructose. Corn syrup is similarly proportioned.
This is also supported by the fact that scientists have already established that elevated insulin levels are correlated with lower testosterone. However, the researchers in the above study found that testosterone seemed to fall independently of insulin levels suggesting that it was blood sugar, i.e. glucose levels that actually depressed testosterone. For example, blood sugar levels are best measured by what is called the hemoglobin A1c test. This A1c test has shown that the worse the blood sugar, i.e. blood glucose (in diabetics) the lower the testosterone.  Therefore, it is very likely that any sugar or high glycemic meal will lower testosterone to varying degrees for hours afterwards. Therefore, the best advice we have at this point is to play it safe and avoid all sugars and simple carbs if you value your testosterone.
For additional protection, there are a couple of other things that you can do:
1) Vinegar. As I document in this link on How Vinegar Can Help Weight Loss, vinegar has a profoundly positive affect on blood sugar metabolism and decidedly dampens the body's glucose and insulin responses after a meal with significant carbs. Just have a tablespoon or two with any carb meal and you will lower your blood glucose and insulin rises post-meal, which should help to protect your precious testosterone.
Anti-Viagra: Glucose (and therefore sugar) now has been shown to take down your testosterone. You should keep in mind that sugar assaults your sex life in many other ways as well. I intend to do a page or two on this sometime but haven't had a chance, but I will summarize by saying that sugar also does a full frontal attack on your erections. Remember: sugar is half glucose AND half fructose and the fructose component directly lowers Nitric Oxide. That's right: fructose directly affects the liver and hits your erections right where they count. They really should call sugar and corn-syrup Anti-Viagra!
It's hard for me to think of too many things that are worse for your sex life in the short term (and long term for that matter). Sugar and corn syrup wreak immediate havoc on several key components of your sexual strength and function. These nasty effects can last for hours and all for what? Sugar and corn syrup add little to no nutritional benefits - in fact, quite the opposite.
Quite a few guys would be reluctant to admit it but have a strong sweet tooth, not realizing what a profound effect this can have on their sex life. Remember that sugar, almost immediately upon hitting the taste buds and saliva, creates an endorphin rush. What I'm trying to say that sugar can make you no brighter than Pavlov's dogs if you're not careful. For a few minutes blast of endorphins, you can basically castrate yourself.
I freely admit this is one of my biggest areas of weakness. It is easier for me not to eat than to eat without at least some sweetness in my meal. It tooks weeks of work to break this addiction but it is well worth the effort!
By the way, I just want to point out that, once again, Jack LaLanne was way ahead of his time. As usual, he foresaw about 75 years ahead of his time these research results for how horrendous sugar is for the body. He has for decades preached against sugar and has never consumed a gram of it since he gave it up in his teens. Undoutedly, this is yet another key to his good health and fitness.
2) Cinnamon. Cinnamon's glucose lowering action seem to be based on methylhydroxychalcone polymer, or MHCP. MHCP is actually a mock insulin and ushers glucose into the cells in much the same that insulin does. One study gave diabetic patients 1, 3 or 6 gram daily doses of cinnamon. Even at the 1 gram dose, the results were particularly impressive: particpants after six weeks of use had glucose levels reduced by 16%.  By the way, it should be mentioned that cinnamon also seems to improve insulin sensitivity , which is one of the hallmark symptoms of a long-term testosterone killer: Metabolic Syndrome. Again, this has not been proven by any study, but cinnamon should help protect your precious testosterone post-carbs just as Vinegar does. CAUTION: There is a certain type of "mock" cinnamon (Cassia) commonly sold in stores that has significant couramin, which is known to cause liver and kidney problems. You only want to buy true cinnamon, which is Ceylon Cinnamon.
1) Diabetes Care, 2003 Dec, 26(12):3215-8
2) Diabetes Res Clin Pract, 2003, 62:139-48
3) Diabetes Metab, 2004 Feb,30(1):29-34
4) Endocrine Abstracts, 2007, 13:P286, "Effect of Glycaemic index of the diet on salivary cortisol and testosterone levels in females"