There are a lot of different techniques you can find by googling. This situation happened to me once, and I succeded using a technique that surprisingly I cant' find anywhere:
I put a very small amount of loctite on the top of a flat barrel of a diameter slightly smaller that the tap, and carefully introduce it into the hole until it makes contact with the broken tap. Then I wait 2 min while maintaining the pressure to make sure the two are glued. Then I unscrew gently...voilĂ .
Problem is you can glue everything together, and then you are screwed, but if you are careful enough, and have a steady hand, it should work.