The answer is going to sleep, at a regular time. Put down the computer, put down the cell phone, put down the games. Go to bed early. Above all else I make sure I get my eight hours every night. If I'm tossing and turning, I'll call into work. As a result, I am NEVER sick. Ever. Anecdotal sure, but if I don't get sleep, I feel like I'm coming down with something.

Also naps, take advantage of them. I'll often go home on lunch, and take a snooze for an hour.

A large part of it is schedule/balance. Before I could get sleep in any condition. But now if I eat too late, I will have acid reflux at times. If I don't have physical activity I'll be too anxious. If i delay sleeping time I'll go off and sleep during the day.

Don't eat late, cut anything too mentally addictive (socnet, binge watching). Relaxing env. Eat well, move a lot (I never sleep better than when Im drain post jogging).

It won't cure all, but even in bad times (I am just past one) it makes a good difference.