Ok this is somewhat common and really it depends on how your masterbating, to porn or your mind. If to porn try making sure your watching long videos insted of short clips, having to do anything but please yourself will no doubt create issues like this. Also if your sitting upright when masturbating try doing it laying down.

[nb, laws state you can't view porn but well, we all do!]

I think it's only psychological problem. I think you just have to relax and to understand that you HAVE to go through this. When you get close to orgasm just continue masturbating and give yourself a relief. When you go through this once, it will be easier next time.

ok thanks for the replies but its not anything to do with the technique - everythings fine until i'm about to come, and then, even when i try to relax, as soon as i feel anything is coming out, i push it back in as a reflex
i have gone through with it a few times but it doesnt seem to get any easier

do it again and again...

anon44,
Did you say that you have never cum through masturbation? You only cum through wet dreams? If you can cum at all then it means that this is physically doable. The reverse would be true that if you couldn't cum, ever during a wet dream then you may have some sort of blocking occuring, but I don't think this is the case. The other suggestions here re this being psychological may have some merit.
When you masturbate, do you find yourself feeling emotions other than "lustful horniness"? What I'm trying to get at, is whether there is something else on your mind, or perhaps when you first started masturbating you did have something bothering you or occupying your thoughts thus causing you not to shoot. Once the trend started, it could now be the anticipation of nothing happening that continues to stymie you in your pursuit to ejaculate.
You should try to relax and enjoy masturbating. Get some good porn (or whatever turns you on) going and, has been suggested, keep masturbating until you ejaculate - don't stop when you think you are about to blow, but instead, keep going until you do so.

Hope this helps,
Wendy