Undo On A Slider!
If you apply a technique (such as a filter or a paint stroke) and the effect is too intense, you can always undo the effect by pressing Command-Z (PC: Control-Z). But if you just want to decrease the intensity, instead of completely undoing it, go under the Edit menu and choose Fade. Want a less intense effect? Just move the Opacity slider to the left. The farther you drag, the less intense the effect. Drag all the way to the left, and the effect is undone.