If you are to learn only one skill, this is it. Mastery of the art of brinksmanship is the single most effective and pleasurable technique for increasing sexual stamina. Literally, brinksmanship is the art of bringing yourself to the 'brink' and then back again, while remaining in control. A master of brinksmanship can either delay the impulse to climax as soon as it begins, or bring himself right up to the very point that one more stroke would put him over the edge. In either case, when he feels himself reaching the threshold (the brink), he begins a cool down mechanism.
The cool down can be one of 4 things...

SLOWING THE PACE
As it sounds, sometimes all it takes is slowing your penetration pace in order to get back your composure. This also helps you settle into another rhythm. This method is least likely to interrupt your girl's progression to orgasm.

LIMITING THE DEPTH
Changing the depth of penetration is a simple technique that is still highly effective. This is generally a good idea to mix in every once in a while even if you aren't on the brink, as the motion of it is irresistibly tantalizing for your partner, and will inevitably lead to increased excitement on her part.

COMPLETE STOP INSIDE
This is a defensive technique. You have gotten a little too far, and if you aren't careful, you could begin the irreversible countdown to ejaculation at any moment. Thrust all the way inside so that your pubic bone is pressing up against her. Make sure that you maintain contact even if she starts to buck her hips back and forth. You want to minimize friction as best you can. If possible, you can grind in a circular motion to stimulate her clitoris, so as to maintain her heightened levels of excitement.

TEMPORARY CESSATION
A last ditch effort. This should be avoided if possible, but sometimes, it's the last option you got. Pull out until only the tip of your penis is inserted in her vagina. The cool air should help cool you down. Pretend that you are teasing her. When you start to gain control, test the waters with a few shallow strokes before you dive back in.
How close you let yourself get, and your level of excitement will determine which method to use. The theory is simple, but the execution takes a great deal of knowledge of your own body, and a lot of mental discipline. Practice all of these methods and learn to listen to the signals of your body so you will never be caught off-guard.