It's hard to believe but I need more undo's in Photoshop. I think the History palette is the best thing since sliced bread but, there are times when the default of 20 undo's doesn't take me back to before I messed up.
Here is one way to fix this.
In Preferences select Performance. Use the slider or type in the number of History States you'd like to have.
Keep in mind that the more History States you add, the more memory you'll need for your computer. You can add up to 1,000 History States but I wouldn't go over 50 myself.
If you find your computer running slow, reconsider changing the number of undo's back to a smaller number.