Kolibear,
A "been there, done that" answer to your post. Break the canopy before you undo your straps. Clearly from the discussions above, lexan canopies require a bit more thought; but perspex breaks very easily particularly when you are frightened and can smell avgas.
Before you undo your straps, put a hand up and brace it against the roof. Otherwise, as you say, you can hurt yourself as you fall out of your seat.
When I was in this situation, I think it would have been very much harder to break the canopy had I undone my straps first.