I've had this happen a couple of times, for a couple of different reasons.
The first, was when i was just getting into sex, and the act of putting on a condom killed my erection two thirds of the time (major pain in the ass). That one we solved by switching birth control methods (we both know that neither of us has any std's).
More recently this has happened a few times, when we start having sex a second time, shortly after the first. If i don't get a fairly large amount of stimulation, it just goes limp. I've found three things that help, and here they are:
1) Find the muscles that control your erection (the same ones you use to stop pissing) when you feel yourself going soft, or while your putting a condom on, or whatever, clench these, and hold them. You will probably have to "work them out" for a while before you can hold them for any length of time (next time you take a piss, practice stopping it mid stream). These are the ones that the semi-famous "Kegel" exercises strengthen, do those anyway, as they after a while, make any sex better, for both parties.
2) Think about, or picture what your doing with, about to do with, or was doing with, your girlfriend (in turn you on the most, give yourself a imaginary porno to watch)
3) change positions. If your not getting enough stimulation, switch to something you get more in, missionary is one of the better ones for guys, though not so great for girls. Think back to porn, and try some of those (you'll probably want to try them before using them to combat loosing your erection, as switching and trying to figure it out isn't exactly stimulating).
I hope this helps!
--Bobnova
P.S. the hardest part is not worrying about the fact that your loosing the erection, which is the one thing you shouldn't be thinking about, naturally.
Bobnova
Posted: 30 Sep 23:52