I use this with great results...

Amazon.com: got2b Powder'ful Volumizing Styling Powder-0.35 oz: Health & Personal Care

You shake it into your roots and sort of rub it in, and instant volume! Now it does leave a residue at your roots that you have to wash out in the shower later, but I don't really care since it works so well and nobody can tell but me. Also, I bought mine at Walgreens rather than Amazon and paid about half the price. The small bottle lasts forever and ever too.