One of the most important things you can do to increase testosterone is to loose weight. Males tend to absorb estrogen in the fat cells.
Now here is a statement I'll get flamed for:
I lost 35 pounds on the Atkins Diet and my testosterone went up from 325 to 455 in 9 months and blood presure and cholestrol was down too. So based on my experience losing weight and high protien helps the process.
There are many problems for middle-aged men with low testostrone so it is something to attend to.
Well....I'm actually 21 years-old (about a month away from 22 years) and I do try to be physically active in whatever environment I'm in.
I actually gain weight in the colder months even though I cut my calorie consumption down to around 2000 calories a day. In warm weather when I'm cycling a great deal, I'll eat anywhere from 4000 to 5000 calories a day, with 90% of it being carbs, and I'll still manage to lose a pound or two here and there.
I can't count the times I've come across folks who have just bought a bicycle to help them lose weight, and who are also on Atkins. The results are almost always disasterous.