INSTANT OPACITY CHANGE

Any time you want to change the Opacity of the layer you're currently working on, just switch to the Move tool and press a number key on your keyboard: 4 = 40% Opacity, 5 = 50% Opacity, etc. If you want an exact percentage, such as 52%, then type in 52 (note: you have to type quickly, or you'll get 50%, then 20%).