Expensive, but appears to work
In the summer, I air dry my mildly natural curly hair which gives me fullness.  But during the Chicago winters, I need something to give my hair more body.  I have several cheaper volumizing shampoos, but this one seems to work the best.  It also doesn't dry out my hair, like some volumizing shampoos can do.  It's expensive ($27 for 8 ounces), but since I only wash my hair every three days, the bottle lasts awhile and subsequently reasonable.
Rating: 5 stars