I would suggest that the only down side to masturbation is that it trains your body to reach orgasm faster than you will want to once you are having sex with someone else. Then, you're going to want to hold off exploding until your partner is also, or has already orgasmed. Why rush it??

That said, concentrating on the sensations you are feeling as you masturbate, as well as picturing fantasy or memory images that turn you on should do the trick. Generally speaking, the less you are distracted from the task in hand, the faster you will cum.