I have the same problem. Users click and re-click because they are not sure if the click took.
Two simple changes would make me very happy.
#1) Make any button or menu clicked change to a blinking word "Processing..." and change it back when the process is completed. This would leave no question in the users mind they clicked the option. So the button "Submit" would change to a blinking "Processing.." if clicked.
#2) And the most helpful suggestion would be to place a RED blinking "Processing..." right justified on the status bar anytime a menu item, button, dynamic record drop down, or any process is started and clear the process wording once the item is displayed would be very helpful to me. Most of the time it is so fast this doesn't apply, but for the times it takes longer for whatever reason this would be VERY HELPFUL and help 99% of my users and stop support calls to us.
I would also suggest a "Turn off 'Processing'" message on the General Tab of the Visual Perspectives just like the Turn Off 'Powered By' so if someone does not want this feature they can turn it off.