Signing up to the Social Club and linking your account will give you a few extra hair/beard styles. You will also have to stalk the companies on Lifeinvader through Social Club, rather than in-game, but that only gives you a discount.
The two different barbers offer different styles - Herr Kutz is all over the place, and Bob Mulet is in Rockford Hills. Bob Mulet is the place to get Trevor with a bald head if you wish.
@Nosyd - Upon switching characters, Trevor occasionally wakes up with nothing but his tighty whities on. All you then have to do is re-dress him without putting a new shirt on. I don't think there's a specific way to remove your top garment.