How do i add volume to my hair?

My hair naturally is wavy. When it was short it used to hair loads of volume, but i'm growing my hair now so it is starting to loose it's volume, and it just sort of hangs there like i've been dragged through a bush or something. I've tried using a diffuser thing on the end of a hair dryer as well, but that didn't really work because it just sort of dried it. So basically, please can you suggest a way to get volume? But not really just with mousse, as that normally gives it a greasy crisp look, which isn't really what i'm looking for.