The best way to combat premature ejaculation is by learning to control the sensations prior to orgasm. This method takes time and practice, but it is very effective.
First you need to bring yourself close to orgasm (this can be done via masturbation, without the involvement of your partner) and then stop and relax before recommencing. Each time you need to bring yourself closer to orgasm until finally you cannot control it. If you do this often enough, you will learn where your point of climax is. This is helpful when interacting with your partner.
You will need to practice reaching your climax point with your partner by engaging in non-penetrative sex so that when you feel it is near, you signal them to stop and you allow your erection to subside. This also needs to be repeated so that you and your partner are familiar with the procedure.
Once you feel you are ready for intercourse, it is best to start by lying on your back so that you can guide your partner during penetration. When you are near orgasm, give your partner a signal to stop and you should relax and start again. Once you get the hang of it (it may take several weeks or months), premature ejaculation shouldn't be too much of a problem.
A variant of this method involves the partner squeezing the tip of the penis just before orgasm ("squeeze technique"). This pushes blood out of the penis and reduces the erection.