Roofs? Easy. Climb at a tall overhanging gym. Get someone better at it to watch. Climb outside at the Gunks! Yes, what Dana and everyone esle has said.

You want the confidence to do it and exact knowlege of how hard you can push yourself while doing it. As in any climbing, mental insecurity can shut you down very quickly. As RG wrote hand strength is important. Really, for fast progress find someone who understands movement and climbing to watch you and advise.