1. Be Here Now
Concentrate less about the finish and more about the journey. Focus on pleasing your partner for a more satisfying experience.
2. Foreplay Counts
If you're timing the length of your performance (which you shouldn't be) add the foreplay time to the total experience. Penetration isn't the whole show. So don't short change yourself by thinking "was that too fast?" Foreplay can be pretty fucking hot too.
3. Stay Fit
Here's another reason to hit the gym. If your body is out of shape you can't expect to go all night between the sheets. Being in shape helps keep the fires burning longer...
Practice makes perfect. Every performer rehearses before the show. There are some amazing sex toys on the market that can help you build stamina. Give them a try but just don't fall in love.