Pace yourself. I figure you probably know how much stimulation for how long gets you sore? Make sure that you don't try to approach that limit, and take frequent breaks. Our bodies have so many erogenous zones, it would be a waste to just concentrate on our genitalia only!

Use a lot of lube. A LOT. I'm not saying that you should be swimming in it, but use it very generously.

Sex in bathtubs, jacuzzis, and shower stalls, while cool, are quite hard on us because water washes away most sorts of natural and artificial lubricants. Be aware of that, and use lube that does not get washed away by water, like Astroglide.

But seriously, pacing yourself will be the greatest help. It's like running. You wouldn't sprint over the quarter of a mile on a 5k run, wouldn't you? It may be hard to keep that in mind in the heat of the moment, but it will be of the essence if you don't want to tap out sooner than you would want to.