Is that Delta?
All airline miles I knew will expire given no-acitivity for xx months. That are many ways to extend, just check out the "earn miles" section after you log in your airline mileage account. In most cases, you can simple do a bit on-line shipping (clicking though the links).
For Delta, you may also join an 'e-miles' program which accumulate miles by answering very simply surveys email, miles transfer to delta account every 500 miles. Another program called e-rewards, similar but survey are much longer and Idt worthwhile.
For United and AA. mypoints is another valuable program that can redeem reward in the form of airline miles.