Here’s a nice trick I picked up from Augie DeBlieck Jr. at LearnCSP.com.

If you’d like to alternate quickly between two tools — for example, the pen tool and the eraser…

…That’s annoying, though, because you’re going to be constantly hitting your keyboard shortcut for your drawing tool and the shortcut for the erase tool….

If you hold down the keyboard shortcut, you’ll have access to that tool until you release the key.  So you can keep drawing with your pen tool, hold down the “E” while you erase, and then take your finger off the “E” to get back to the pen.  It’s super useful and super easy.  After you need it once or twice, it’ll be muscle memory.