Summarize this article:

Fluff pillows daily to maintain their shape.  By fluffing the pillows daily, air can circulate to keep them from getting permanently deflated. You won’t have to replace them as fast. Wash your pillowcases regularly too. Who wants to sleep on a dirty pillow case? Washing your pillowcases every week is a good idea. Some pillows cannot be fully restored and should be replaced.  Experts suggest replacing pillows every 4-6 years, primarily for removal of dust mites. Place your pillow on a flat surface. If it has lumps, bumps or dents, it’s probably time to replace it. If you wake up with a sore neck, it’s probably not providing you with enough support. Once pillows have been repeatedly crushed and the stuffing is flat due to moisture (likely sweat), you probably won't be able to return them to their original fluffiness. If the pillow smells of mildew, replace it. Fold your pillow in half and then see if it pops back into place. If it doesn’t, it’s probably time to replace it.

Summary:
Regularly fluff your pillows. Replace your pillow every few years.