Deep and steady breathing.
I also find that my staying power varies with my physical condition. Might be related to how horny I am, I dunno. Sometimes that "muscle" feels stronger, sometimes it feels weaker. I would try to determine where my limit is, go slow initially and test for it. Once I know where my limit is, it's easier to pace myself to keep going.
I also try to think of some unsexy stuff like work, lol...