Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
Tapping the retry button instantly takes you back to the start of the level.
If a rock bounces into your mouth, you can catch it and use it again.
Always be on the lookout for ricochet shots, for they're the quickest way to clear a level.