1.      Try to stop closer and closer to the point of no return. Get a good feeling for this point of inevitability, so you won’t cross it.
2.      In stead of stopping, make subtle adjustment. You could for instance slow down, stimulate a less sensitive part of your penis, try a different kind of stroke, use your other hand or stay away from the glans. Start these adjustment a little earlier than when you normally stop.
3.      If step 2 works well use saliva or a lubricant. First exercise with stopping. If this works well you can make subtle adjustments instead of stopping.
4.      Do the same exercise, but fantasize about sex while you masturbate.

It takes time to develop control. Normally it will take some months, practising three or more times a week. So don’t give up too quickly!