Most dry shampoos will act as a volumizer. Be sure to only use it on your roots, wait a few minutes, scrunch, then gently brush it out. I saw this process described, and good products recommended to use, in the February 2011 Lucky magazine. Try looking at Sephora.com for ideas, and try them out--Klorane makes a really great one, costs $15-$22. Don't use a cheapo one, it'll just turn your hair grey!
I don't know what kind of shampoo you use but the cheap stuff in stores weighs your hair down with wax- try salon brands of shampoo and conditioner that add volume- if you don't use salon brands, you will be delighted at how luscious your hair is- ANY salon brand will do, just don't use regular store shampoos no matter how expensive or what claims they make