I'm prone to mild acne breakouts and recently I've been using Neutrogena Daily Scrub oil-free acne wash. This stuff is awesome.
Not only does it seem to actually
work, but it makes my skin feel awesome after use. The stuff has small sand-like particles in it and so it literally is a scrub. I shave in the shower, and after a shave I put about a teaspoon of this stuff on my head and just scrub and rub the hell out of my scalp. The sensation afterwards is hard to describe because I feel so clean and the skin on my head feels soft... I dunno... it really is hard to describe.
There are other varieties of this kind of stuff, but if it doesn't have the grit particles in it then it may protect you against acne but it will not give you this satisfying cleaning effect.
And, yes, it works best on a freshly shaved smooth head. When I use it on stubble it just doesn't seem to work as well.