Since you don’t have unlimited undo’s in Photoshop, you have to use the History palette to step back farther. The quick way to do this, and a way which seems really similar to the unlimited undo feature of Illustrator and InDesign, is to use Option+Command+Z. This lets you step back through your history to the beginning of the recorded history. Remember that your History palette by default has a limited number of steps it will save so if you try to go too far back it won’t let you.
If you want to change the number of actions your History palette will remember, you can go to Photoshop>Preferences>Performance and adjust the History States slider. I wouldn’t advise doing this unless you really think you are going to need more than twenty stages in the History recorded since it will slow your computer down considerably once you have worked through many of these stages.