Practice getting to the edge of cumming, but then do something to distract yourself, and do not cum. Doing this over and over will build up your resistance and skill. Also, there are herbs that can be taken as tea that will also help. Schizandrae is one for instance. Go to a Chinese herbalist and ask for a formula. Practice is key though. No herb will do all the work for you.
By delaying your orgasm over and over, when you finally do allow it, the result will naturally be more powerful. Especially if you can work on getting to climax, but not cumming, over a period of several days.