*insert fanfare here*
Karate chop! That's right! Fluff your pillow up really well, and then give those little babies a good karate chop on the top with your forearm. If your pillows don't maintain the chop, I'm sorry to say your pillow inserts are probably not down and are definitely not good quality.
I do this with all three pillow that I usually keep on my bed. I also do it for the corner pillow (on my pouf) and the one in my desk chair.
This simple trick is fun and really will give your pillows that extra umph!