Remember those post-puberty glory days and the change in facial terrain that followed? Well, some guys find themselves observing the same phenomenon after boosting their testosterone via Hormone Replacement Therapy! Yes, that's always fun to explain to the wife and kids, eh? Actually, sometimes it is not as visible to the general public. I have noticed on The Peak Testosterone Forum that many guys get acne on their neck, back and shoulders.
How does testosterone do it? Many studies have shown that increasing testosterone levels increases DHT (dihydrotestosterone) levels, which in turn fires up the sebaceous glands. 
However, one thing most guys don't realize is that diet alone can help very significantly with acne and I'll cover that below and some of the key dietary scientific findings, mostly from the last four years (as of this writing) that can help or even cure this issue. I even through in a standard cosmetic treatment that may help as well:
1. Low Glycemic Diet. A couple of studies have shown significant decreases in acne through a diet based on low-glycemic carbohydrates, such as most whole grains, fruits and veges.  One of these studies was based on fairly high protein (25%), medium carbohydrate (45%) and medium-high fat (30%).  Care should be taken to avoid too much saturated fat in order to avoid erectile issues, which means the protein would need to come probably mostly from egg whites. See my page on The Potential Dangers of Saturated Fat In Men (particularly in men over 40) for more information.
2. Fish Oil. Multiple studies have indicated that increased consumption of fish and sea food, and thus most likely fish oil, is associated with decreased acne. 
3. Red Light Therapy. A recent study showed that red light therapy appears to be an excellent solution for acne. Acne is primarily caused by bacteria that live within skin pores where red light therapy specializes in penetrating.  You may recall from my page on Skin and Eye Treatments that red light therapy is also very effective aginst the wrinkling that occurs around the eye, a difficult are to treat in general.
4. Dairy. Some people reports good results with acne simply by giving up dairy.  This is anecdotal but is easy and worth a try. I would recommend taking some supplemental calcium to make sure that you get enough.
5. Stress. Watch your stress levels: cortisol accelerates acne by altering hormone levels that jump start sebaceous gland oil production. Read my link on How to Reduce Stress for research-backed ways to decrease cortisol levels.
6. Diary and Milk Products. Finally! Dermatologists have for yours denied a link between diet and acne. However, a recent review recently overturned this assumption and found a consistent link between dairy consumption and acne. 
7. High Glycemic Meals. The same study mentioned in #6 above also found a link between high glycemic eating and acne.  The key concept to understand is "glycemic load". Some foods are high glycemic foods, such as carrots, but you would have to eat so much of them that it they never spike insulin or blood sugar significantly. For the sake of your skin, watch your glycemic load. Yes, we need carbs, but they need to be whole food, high fiber.
8. Clearasil. Although not exactly natural, some of the over the counter treatments pretty low in side effects. (Discuss with your doc or pharmacist of course.) And I'm not really advocating one brand, but the point is that sometimes you can use the standard "high school" over the counter treatments to get things under control. One poster on the The Peak Testosterone Forum wrote: "I started using Clearasil Stayclear twice a day and it is already noticeably clearer."  In addition, there are a lot of over-the-counter preparations that help with acne and they usually contain various combinations of salicylic acid and benzoyl peroxide. Clearasil's most potent formulation is 10% behzoyl peroxide: Clearasil Acne Control Vanishing Cream, 1 oz is an example. Other Clearasil products contain salicylic acid, generally 2% in strength. (I have not seen greater than that.) Both of these ingredients are proven acne fighters, but sometimes you need with proven wrinkle-fighter Retin-A. (If you put it on your face, you have to be very careful with sun exposure. Discuss with the doc that writes the prescription.)
NOTE: Acne tends to flare up with anything that dries out the skin. The reason? If the protective layer of oil on the surface of skin is removed, many guys' sebaceous glands kick into overdrive pumping out more oil. So cleaning your face with harsh shampoo, a dry, summer day - all of these can put trigger or exacerbate acne.
1) J Am Acad Dermatol, 2007 Aug, 57(2):247-56, "The effect of a high-protein, low glycemic-load diet versus a conventional, high glycemic-load diet on biochemical parameters associated with acne vulgaris: a randomized, investigator-masked, controlled trial"
2) Am J Clin Nutr, 2007 Jul, 86(1):107-15, "A low-glycemic-load diet improves symptoms in acne vulgaris patients: a randomized controlled trial"
3) Arch Dermatol, 1961 Dec, 84:898-911, "Adolescent acne and dietary iodine"
4) Photodermatology, Photoimmunology & Photomedicine, Published Online: 11 Sep 2008, 24(5):244-248, "Non-invasive diagnostic evaluation of phototherapeutic effects of red light phototherapy of acne vulgaris"
5) Prevention., April 2010, p.4.
6) Journal of the Academy of Nutrition and Dietetics, 2013, 113(3):416-430, "Acne: The Role of Medical Nutrition Therapy"
8) Journal of Investigative Dermatology, 1992, 99:509ï¿½511; "Control of Human Sebocyte Proliferation In Vitro by Testosterone and 5-Alpha-Dihydrotestosterone Is Dependent on the Localization of the Sebaceous Glands"