Oil is your enemy. Shampoo daily to get rid of the grease that can weigh down hair at the roots; dermatologist Ciraldo loves Neutrogena T/Sal shampoo ($7.50) for oil control.
And go easy on conditioner; hairstylist Eva Scrivo recommends using one with a gel consistency (like KMS California Add Volume Gel Conditioner, $18) to keep hair smooth without flattening it.