If your FAA pilot certificate is printed on paper, it's going to expire on March 31, unless you replace it with a new plastic certificate. To get the new counterfeit-resistant certificates, you have to fill out a form and mail it to the FAA in Oklahoma City along with $2 for each certificate you want to replace, or you can do it online. Either way, the new certificate won't list your original date of issue, so you might want to keep that old dog-eared piece of paper to prove your longevity. If you apply by mail, it's going to take four to six weeks, and up to 10 days for online processing, so don't put it off till the last minute or you could find yourself grounded. Some non-pilot certificates, such as those for flight engineers and mechanics, are good for another three years but then they will also have to be replaced. Student certificates are not affected.
While you're at it, you can also ask the FAA to issue you a new certificate number that is not your social security number. There is no additional charge to make this change. Click here for more info and the forms you need, or to make your request online.