I always use a hairdryer, you can regulate the heat with settings and the distance from the work piece and duration of heat applied. Can also go back to tweek if need more movement, a little at a time is best so as not to crack surrounding areas. Also heat surrounding areas slightly ( ie: a body) for the same reason and also to help these areas change their memory also.
I have noticed resin tends to need way less heat than styrene, again a little at a time.
Also, if warping isn't bad, when finished with each round of heat, hit with some cool air. If working with a heavily warped piece, you can let heat come down more slowly, then maybe some cool air. Just play with it to develop the technique.