Yeast and hops help to swell the hair shaft and plump the cuticle, adding volume.
The acidity of the beer helps remove built-up product residue. In the shower, after you've shampooed, pour a bottle of beer over your hair. Rinse briefly with fresh water (the idea is to avoid the Eau de Barfly effect while retaining the beer's body-boosting benefits). A rich beer with a high yeast content works best. (Read: No light beer.)