Change the Fill Opacity on the fly
You probably know that (with a non-painting tool selected) you can press a number to change the opacity of a layer (1 for 10%, 2 for 20%, etc.). But what if you want to change the Fill opacity rather than the opacity? Add in the Shift key, pressing Shift-1 for 10%, Shift-2 for 20%, etc.
This is an awesome quick tip! I ran across this awhile ago and was wondering why my opacity keeps changing. To add to this tip even further, if you want to change the opacity to 17, just press 1 and then 7 quickly. I thought it was only in increments of 10, but I was wrong. 🙂