#1, it's all in the editing...anytime they change camera angle, the rest of the scene could be shot an entirely different day.
#2, they probably blow a load or two before shooting a scene so that they can last longer.
#3, they probably shoot all of their scenes for a movie in succession (i.e., one after the other, just enough time to put on different clothes). By the end of the day, having an orgasm is probably as daunting a task as holding one back was at the beginning.
How can you accomplish what they seem to? Practice. Get older. Practice some more. Teach your girl that when you slow down/stop, doesn't mean it's time for her to start. Honestly, women who complain about guys cumming too fast are often to blame for it. Oh, did I mention practice?