1. Use It or Lose It

You need to have erections regularly to keep your penis in shape. "It has to be essentially exercised," says Tobias Kohler, MD, assistant professor of urology at Southern Illinois University School of Medicine.

To maintain a healthy tone, the smooth muscle of the penis must be periodically enriched with oxygen by the rush of blood that engorges the penis and makes it erect, Kohler says.

If a guy is physically able to get erect, but never has erections during the day -- maybe he finds himself in very un-erotic circumstances for a long time -- he needn't worry. The brain has an automatic penis maintenance function built in.

Impulses from the brain cause erections during the dreaming phase of sleep, called the REM phase. It doesn't matter if you're having a hot sex dream or a zombie apocalypse nightmare -- your penis gets hard during that period of the sleep cycle.

But some men are physically unable to get erections, such as those who've suffered trauma to the nerves involved or who have nerve or blood vessel damage caused by diabetes.

"If they don't do anything to maintain normal erections, they will get shortening of the penis," Kohler says. Without regular erections, penile tissue can become less elastic and shrink, making the penis 1-2 centimeters shorter.

A device like a vacuum pump, which forces the penis to swell with blood, can help men with physical erection problems maintain a healthy penis, Kohler says.