You can use CTRL + ALT + Z for multiple undo's
So hold CTRL + ALT and keep pressing the Z key
Thankyou, I should have mentioned I do have a Mac, so I do not have the ALT button!
Ctrl + Alt on a PC = Cmd + Option on a Mac
As well as the multiple undo methods described above you can also use the history panel to jump back several steps in one click.
Ctrl+Alt+Z (Windows) = Ctrl+Option+Z (Mac)
You also can use the History panel (menu Window).