I just started a thread about a similar problem but from a male point of view:
Intense desire to cum

There comes a point after a few minutes of thrusting where the penis becomes desensitized and ejaculation requires extra forceful thrusts, I found that a long blowjob can help get to this point, otherwise be patient at the start when he is trying to control himself and tell him to start very slowly until he reaches that point.
The sexual position also can have an effect, missionary and other postitions which tense up muscles accelerate orgasm whereas you on top will delay it.