There's really no good excuse for letting your hard earned frequent flyer miles expire due to account inactivity (in case you didn't realize, most airlines will expire miles after 18 or 24 months if you don't have some "qualifying activity" with them--such as taking a flight, but also such as buying something through their online shopping malls or renting a car, staying in an affiliated hotel, etc.). As we've pointed out in the past, if you buy stuff online the easiest and most rewarding way is to get bonus miles by purchasing through the airlines' shopping malls.
Other easy ways to keep miles active include redeeming magazine subscriptions from your rewards account. They are as cheap as 400 miles with most airlines at www.magsformiles.com. Or join your airline's dining for miles program if they have one. Some people we know just buy a slice of pizza or a cocktail at a participating restaurant and their miles are good for another 12 or 18 months. It's easy and rewarding. And of course, if you have an airline-affiliated credit card, you're golden.
If your miles have already expired, here is some information on how to reactivate your miles. Yes, it will cost you money, but it may be worth it depending on how many miles you have. It is always smart to know what you want to redeem your miles for before reactivating them to insure there is availability first. This chart will show you how and what it costs. AirfareWatchdog is always here to help answer your questions if you need help.