Well there might be a couple of solutions for you, maybe some of them already mentioned here.
First of all, sex isn't all about "cumming". I know for the guy it may seem so, but most of the time the girl is still horny after you "cum". Try to cum more then once, for example after having your first orgasm, start all over with foreplay, eat her out maybe, caress here, caress there, then when your manhood is stiff again go for it.
If you are looking for something more, something you could actually feel changing in you, then you might want to try to work out your "pc" muscle. Now i'm not an expert at that stuff, but I think i've mastered it myself. Basically you "squeeze" a muscle down there "x" amount of time each day and, well I can't explain it but your orgasms will change and you will last longer (i.E. You will control when you wish to "go"). So, next time you go pee try and stop peeing all of a sudden in the middle of it all. That muscle you "squeeze" in order to stop peeing is the one you have to concentrate on. Just do like 3 reps of 15, holding it for 1 sec. Each time and within 1 month you will notice the difference.
I have to go now I was going to write more... I'll give you a quick example of what this could do for you, I had an orgasm w/o "cumming" (you just squeeze and hold when you feel it about to come out). Anywho hope this was of any help..
Peace