I believe relaxation has a lot to do with maintaining a hard erection for an extended period of time. Not too relaxed or you'll lose it... We all jerk off from time to time, so you should know the feeling you get when you're about to explode. When you're fucking and you get that feeling, just change the tempo or the position and continue on your happy way. I usually change the tempo first (slow it down, focus on kissing or massaging for a few minutes) then go. If that doesn't work change the position.
I've also found that if I start with the woman on top, I stay more relaxed and she usually comes first. Then we turn over and can do missionary and/or doggy before I get my first one. There are actually quite a lot of positions for a woman on top... and you get to just lay back and watch the show
