To make sex last longer, relax your PC muscle.
The PC muscle is shown as a thick black curved band in the diagram below:
The easiest way to locate your PC muscle is to stop the flow of urine while you're peeing. The muscles you use to do that are part of the PC muscle group.
By relaxing your PC muscle, pressure is taken off the prostate, orgasm is delayed, and premature ejaculation is cured. The aim of longer sex is to stay away from orgasm.
This helps to keep sex going for as long as both you and your partner desire, and improves the chances of your partner having an orgasm during sex.
You can practice this with your partner although you will learn much quicker on your own first, while masturbating. Learn what it feels like to focus on relaxing the muscle.
Why PC muscle relaxation works:
The technique works because the PC muscle sits right beside the prostate (see the diagram above). The prostate plays a central role in orgasm and ejaculation. When the PC muscle is tensed, it puts pressure on the prostate and brings orgasm quicker. Relaxing your PC muscle reduces pressure on the prostate and helps avoid premature ejaculation. It also reduces pressure on the surrounding organs, the seminal vesicles and the ejaculatory duct, both of which play a central role in orgasm and ejaculation.
Traditionally, sex lasts only a few minutes before the man orgasms. As soon as penetration begins, the man tenses his PC muscle (usually without noticing) because it feels good. Moments later it's all over.
That's not to say you can't orgasm during sex! After having sex for longer you can of course have an orgasm. The longer you leave it, the more satisfying the orgasm will be for you.
The opposite problem: if you go soft:
Sometimes if you concentrate too much on staying away from orgasm, you may get the opposite problem: you start to lose your erection.
This is easy enough to fix: tense your PC muscle (the opposite of what you've been told above). This increases pressure on your prostate, increases pleasure. When you feel your erection is back, you can then go back to relaxing your PC muscle. You have to get the balance right between staying hard and staying away from orgasm, but with some practice this becomes very natural.