If you have Designer Pro 9 you can easily create custom keyboard shortcuts and assign a single keystroke to the Mask Painter tool and the Eraser tool. This makes switching between them while prepping a photo for background removal much easier.

I've been using A for the Mask Painter and X for the Eraser Tool, I find that these work nicely and shift will toggle between adding to or removing from the mask or erasure. I also use the spacebar to pan while I'm prepping.