Think of drawing on layers of acetate. You draw on the first layer. Then add a new sheet and draw some more. Now add a fresh sheet and draw a mask shape. Then on a new layer, you simply draw only in the area defined by the lower mask shape. When your done add another layer etc....etc... You can mask...

I've been playing with this a bit over the weekend. I wanted to take an illustration of a 50lb weight, and drop it on a see-saw type device with boxes on the other end. Now I could trace it, but I would looses the textures and the look I wanted. I tried image textures, but had trouble lineing the up...