first off, don't let the drill spin fast, cos then it just doesn't grip at all.
Drill a new hole in the bolt, bigger than the last, to make a new surface for the extractor bit to grip, then turn it slow as hell, put the extractor in at the same rate as the thread of the extractor goes, and ease it out that way.
If that doesn't work, then find a drill bit that's a size down from the bolt that's stuck in there, and just drill it out, being careful not to tag the thread. Once the core of the bolt is out, use a flat-head screw driver to poke out the rest. It should fall apart with a bit of poking in the right places, and some WD40 etc. Don't use WD40 while trying to use the extractor