To remain as healthy as possible -- both physically and financially -- you should be smart about how you use Medicare. Failing to do so can result in your paying more than you have to or receiving less care than you need. Here are five Medicare rules to live by that can help those approaching or already receiving Medicare coverage.

If you're late, you'll probably regret it.

No. 1: Don't be late enrolling

A key thing to understand about Medicare is that if you're late enrolling for it, you can be penalized -- rather significantly. Here's the scoop: You're eligible for Medicare at age 65, and can sign up anytime within the three months leading up to your 65th birthday, during the month of your birthday, or within the three months that follow. Miss that seven-month-long "Initial Enrollment Period" and your Part B premiums (which cover medical services, but not hospital services) can rise by 10% for each year that you were eligible for Medicare but didn't enroll.