Here is a sure fire trick to getting a dropped chain back on, without getting off of the bike. It has to be done quickly, though.
As soon as you feel like the chain has popped off, put the front gear in the biggest chain ring. It might take a little extra hard push. Keep pedaling and you will feel the chain pop back on.
And, if you have to put the chain back on while you are off the bike, this works, too. Just put it in the big ring and twirl the pedals around to pop it on.