one thing I really use a lot in photoshop is the "preserve transparency" feature in the layer palette. You can paint on a layer with any size of brush. If there is something under your brush, your brush will put some colors, if there is nothing, the brush does nothing. It's really usefull to work on an element of a drawing with a big brush without filling all the layer.
PS : I'm sorry if this feature has already been asked, I havn't read it