I can't believe you guys don't know about this?

You simply jack off until you're about to cum and let go. You can allow pre-cum to come out, but don't actually come yet. Then continue until you feel like coming again and let go. Repeat for like an hour straight and it will build up a ton of sperm and also result in a much better orgasm. However, this is also good training for your penis's stamina.

I did this method and after an hour I let myself come.. and then when my gf came over at night.. we had sex for so long that I made her come 4 times! And she usually lasts longer than me!
Try it.