How to Create Volume and Body by: Hairstyles Teacher
The Hairstyles Teacher has the same issue with her own hair, so this is what I do:
I use volumizing spray - either the Pantene brand or the Aveda brand. I actually like the less expensive, Pantene brand a little bit better for volumizing, but the Aveda brand smells great.
You can also blow dry your hair upside down - making sure that you have sprayed the volumizing spray at the roots of your hair. Make sure you also spray your hair with a heat protector to help protect against damage from heat products.
Finally, curls and back-combing can add a lot of volume to your hair, but again, use a heat protector to prevent damage, and try to only back-comb at the roots of the hair where the hair is stronger.