Dennis,

The one thing I'd like to add that is a game changer is using layer mask in Photoshop. There is no need to ERASE anything and lose it forever. By using the mask you can reveal what is below and if you don't like what you revealed you just switch the color of your brush to white instead of black and paint on the layer mask and you reverse the technique. You can change the opacity and flow of the brush to give all sorts of effects.

Hal