correct me if im wrong but if you broke the bolt shouldnt the head still come off? wont it just slide up? then you can see wats left of the bolt.
http://www.google.com/products?q=screw+extractor&hl=en&aq=f
get the biggest one u can fit on the bolt, use the reccomended drill bit size for the given tap, and be super carefull, if u break the tip of the extractor in the bolt ur gonna be double fucked
if u can clearly see the head of the bolt and ther are no obstructions, use one of these http://www.google.com/products/catalog?q=bolt+extractor&hl=en&cid=17577422353899760981&sa=title#p
^ well 8$ screw extractors are pretty much cheap tools
^ well 8$ screw extractors are pretty much cheap tools