when i first started having sex, i had a hard time lasting as long as i'd like too. i just tried my hardest to think un-sexy thoughts. i'd run through the multiplication tables in my head, count by intervals of odd numbers (7,14,21..., or 9, 18, 27...), and play bad television themes in my head (gilligan's island, perfect strangers...). sounds weird, but it worked for me. eventually you'll build endurance.
also with sex, don't just drive your dick in and out like a piston. variation is important. (from missionary) try starting off by barely penetrating her; back and forth but just small motions. this shouldn't be too over stimulating. do that for a little bit, and then gradually make longer deeper motions. don't go crazy though. don't go to fast or try to push yourself in as deep as you can. do that til you can first detect yourself approaching an orgasm. then take a breath and push yourself as deeply into her as you can. stop thrusting or you're finished. while applying pressure, move your hips in a circular motion. the circles aren't nearly as stimulating for the man. she should still pretty into it though. hopefully you've just stimulated her quite a bit and is pretty sensitive. when you hit that new depth, you should get a big response. also keep in mind that when you're moving your hips, you're rubbing your pubic bone against her clit. if you're still approaching an orgasm, slow down and forget about your penis and focus on your body pushing against her clit. do that for a while. you should be able to compose yourself some. also reverse direction if you're up for it. explore from there.
don't forget to try different positions. i may have just gone in to way too much detail, so i'll spare you some now. if you're interested, let me know. i have one position that seems to work very well for the ladies and is quite easily maintained by the man.