One of the most embarassing conditions that guys can develop, often with aging, is gynecomastia, or "man boobs". Sure, going bald is unpleasant, but at least it's associated with our gender! Furthermore, getting rid of gynecomastia is difficult at best. If a man develops breast tissue, it tends to harden with time and can only be surgically removed.
So how can you prevent developing bigger hooters than the little woman? Most men know that gynecomastia risk is tied into estrogen. As a man ages, he generally tends to have higher estrogen levels and, therefore, can develop this condition.
Of course, that is true, but there are many much more subtle reasons that one can develop gynecmastia and, furthermore, it is often associated with many underlying medical conditions. For this reason, anyone with gynecomastia should get checked out, embarassed or not, and try to find the root cause.
Below are a few of the common things to watch for and prevent. However, I should add that a good working knowledge of testosterone decreasers and estrogen increasers is critical, because any of these could potentially lead to the development of increasing breast tissue.
1. High Prolactin Levels and/or Prolactinoma. As you know from my link on Prolactin and Testosterone, elevated prolactin leads to reduced testosterone, which can skew the all-important testosterone to estrogen ratio. It is this ratio, rather than just one hormone or the other, that is often the problem and has been associated with gynecomastia.  Some pituitary adenomas secrete prolactin and can lead to gynecomastia. 
CAUTION: Further evidence that elevated prolactin can lead to gyno comes from Risperidal, an anti-psychotic.  Risperidal increases prolactin and thus quite often has gynecomastia as a side effect. Risperidal can also increase the risk for prediabetes and diabetes as well. Discuss any such symptoms with your doctor.
2. Weight Gain. It is no secret that gynecomastia is often associated with being overweight or obese. Of course, fat cells as they grow in size have more aromatase, which converts one's precious testosterone to estrogen. Plastic surgeons have even debated whether or not it is worth performing gynecomastia surgery on obese patients. 
3. Hyperthyroidism. About 20-40% of men with an overactive thyroid develop gynecomastia due to elevated testosterone, leading to elevated estradiol-17 beta. 
4. Steroids. Steroid users are in a constant battle againts gynecomastia. As testosterone levels are increased beyond their normal physiological range, more and more estrogen is aromatized and users begin to feel nipple tenderness. Some will even begin to leak liquid from their nipples! To battle this steroid users will often take estrogen-blockers simultaneously or immediately after a cycle as well.
5. Medications. The list of drugs that can contribute to gynecomastia is almost endless. Here's a list from Mayo Clinic: Proscar, Valium, antidepressants, antibiotics, ulcer and/or reflux medications, calcium channel blockers. 
6. Recreational Drugs. Almost every recreational drug that you can think of can lead to gynecomastia: marijuana, alcohol - a notrious estrogen-increaser, heroin, etc.
7. Kidney Decline. Patients with decline and/or dialysis experience an alteration in hormone levels such that they frequently develop increased breast tissue. 
8. Poor Nutrition. Testosterone is dependent on many nutrients and thus a poor diet and nutrition is often associated with decreased testosterone and a skewed testosterone to estrogen ratio.
9. Progesterone. Elevated levels of progesterone have also been associated with gynecomastia. This hormone plays a role in breast development in women and can do the same in men. Read about it here in my link on Progesterone and Men. (See #3.)
1) Skinmed, 2010 Jul-Aug, 8(4):201-2, "Prolactinoma can be associated with gynecomastia"
2) Ned Tijdschr Geneeskd, 2004 Apr 24, 148(17):809-13, "Three patients with gynaecomastia"
3) Med Clin North Am, 1975 Sep, 59(5):1109-21, "Gonadal steroids and gonadotropins in hyperthyroidism"
4) Annals of Plastic Surgery, May 2010, 64(5):688-690, "Adolescent Gynecomastia: Not Only an Obesity Issue"
6) Rev Med Interne, 1984 Jun, 5(2):110-3, "Gynecomastia and chronic renal insufficiency in periodic hemodialysis. Demonstration of a decrease in the free fraction of testosterone"
7) http://www.medscape.com/viewarticle/571416_6, "Use of Risperidone in Children With Autism, Bipolar Disease, or Schizophrenia"