Avoid using a heat gun on a motorcycle helmet at all costs, ESPECIALLY if it is tinted. Blackula727's method is the best I have seen to date, and the one I use myself (the link to the thread is on the previous two posts, so I won't bother posting it). Heat guns on visors have a nack of bending the visor in strange ways or separating the tinting from the plastic material. Motorcycle helmet visors are pretty flexible on their own (they're designed that way so they deflect in the event of an impact rather than shatter), so they can easily fit most helmets. Hope this helps. Good luck.
Edit: Also, avoid using superglue on visors as well. The chemicals in superglue create a white fog on clear plastics, this fog will not go away either.