Well, you could always try getting a little drunk first, which will dull your senses. There have been occasions where I was able to last for 30-40 minutes while having drunk intercourse. Just don't get too drunk or you won't even be able to get an erection at all.
Another option would be to masturbate earlier in the day if you know you'll be having sex later. The longer it has been since you've had an orgasm the more likely you are to ejaculate prematurely.
Alternately, you could get one of those Fleshlight Stamina Training Units, which is designed specifically to help men last longer during sex. I don't know if they really work or not, but the website claims that if you can last for 10 minutes while masturbating with an STU, you can last for 20 minutes with an actual woman.