Do you jerk off regularly? If not, do. And vary your stroke. When you feel close to orgasm, ease up for a bit until the urge recedes, then get back to it. (Bonus: doing this repeatedly leads to a more intense orgasm).
Same with fucking: when you feel close, ease up. Change positions, change depth, change speed--just relax a bit until you're a little more calm.
Don't neglect other ways of getting off. Depending on how quickly you're ready again, try coming before fucking--like from a bj. Then you've got hands and a mouth to please your partner with until you're ready to fuck--which will last longer.