1. To undo more than one history state, use Ctrl + Alt + Z, not just Ctrl + Z
2. View the History window by going to Window > History
3. The default number of history states is 20
4. Go to Edit > Preferences > Performance
5. Set the number of "History States" under "History & Cache" to a larger number such as 50
6. While there can be as many as 1000 history states, this will drastically reduce performance