I don’t know about the space bar, but you can force yourself to use the left shift key by disabling the right shift key. Then after a few hours, once you’ve built up the muscle memory for the left shift key, you can re-enable both and go along your merry way.
To disable (and later re-enable) the right shift key, use
KeyboardRemap4MacBook, and follow the instructions
here.
There are a lot of neat things your can do if you’re into remapping keys. Personally, I have my shift keys set so if I hold them down they act normally, but if I just tap them they type left and right parentheses ().
Also, I set my esc key to trigger caps lock (because it’s out of the way), I set my caps lock key to trigger esc when tapped and control when held (because it is super-easy to reach), and I set my control key to act as command-option-control (so I have quick access to custom shortcuts I make in System Preferences or BetterTouchTool).
I followed the instructions
here and
here (using PCKeyboardHack and KeyboardRemap4MacBook, but not Keyboard Maestro because I use BetterTouchTool.)
Also, I only just now noticed, thanks to this thread, that I also only use my left thumb on the space bar, so as I am typing this post I am doing my best to use only my right thumb on the space bar. It was a little unnerving at first, but I’m getting the hang of it now (though I did just use my left thumb out of habit a couple times in this paragraph.)