Do I love these shortcut keys in Photoshop (CS4)...
Yes I do! There are a couple of keyboard shortcuts I have been recently introduced to. Better late than never!
To undo an effect I used to use the Ctrl Z key combination and get that one undo. Now I use Ctrl-Shift-Z (PC)
or Cmd-Option-Z (MAC)
for multiple undos! Just keep clicking and you now have multiple undos. You don't have to go into your history palette!
My second favorite key combo is to flatten all the layers in your image to enable you to work on the composite as a whole. You may not want to flatten your layered work as you may need to tweak it. I used to select all the layers involved in the composite, duplicate and then flatten and move to the top of the layer stack. This may seem like a simple task but if you work with many, many, many, (did I mention many?) layers of which some don't apply to a particular finished product, it is a daunting task.
The keyboard combination is: Shift-Ctrl-Alt-E (PC) and Command-Option-Shift-E (Mac).
This creates what is referred to as 'stamp visible' layer of all the layers you would like to work on. You now have a flattened version of all visible layers. All of your previous layers are untouched.
A tip here when using stamp visible, be on a layer close to the top of the stack. This will put the complete layer at the very top.
Here is a link to a nice site with a easy view of keyboard shortcuts.