A lot of good replies above. Yes, jerk off before and getting high before sex should help ya but if you want to...you can also practice not cumming while masturbating. Like when you are masturbating and you are about to cum, stop touching and let the feeling of cumming go away and then start masturbating again...then stop, then go at it again.

With a girl, while you are having sex, before you are about to cum, tell her to stop moving or just pull your dic out. Tell her because being in her feels sooo good, you want to make it last longer, and lick her tits for a while or kiss her neck...and then go at it again. Trust me, when you do this, when you finally let yourself cum, you will explode!!!!! You will be twitching. And girls find that sexy. It makes them feel like they turned you on THAT much and it really makes them feel good.

OH, and one more thing, make sure she's satisfied too.