As you are masturbating, the sensation of pleasure will begin to intensify. Keep going, but if you want to experience maximum pleasure don't let things happen too quickly. To prolong your pleasure, you can stop for a minute and give your penis (and your arm) a rest. As well as being fun, this is a very good exercise for later on: you will be a better lover if you can hold back your ejaculation so as to give your partner more pleasure. It will also help you get to know how your body feels when you are approaching orgasm.