I use the alt+click method for clipping masks but the shortcut keys will depend on what version of Photoshop you're using. In the older versions Cmd+G was for clipping, but starting with I think CS2, Cmd+G creates groups and Cmd+Option+G creates clipping masks.
In CS4 you should be able to either use the alt+click method or Cmd+Option+G with your top layer selected. Make sure whatever layer you want to clip to is directly below the top layer.
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