They are based around contracting your PC muscles (the same muscles you use to hold back your urine, if you need to find them) at regular intervals in sets at least 2 times a day. Though my problem hasn't been resolved yet, I have heard about and have started practicing Male Kegel exercises. I'm 18 and I've been having the same problem with the limpness when there is no stimulation. Humans have to learn everything, there is no sexual instinct telling you exactly what to do. You won't ALWAYS be succesful, and if you aren't, don't be that upset. This can be achieved through not masturbating for a while.
Allso try directing your partner to do what you like.
Only you know what really bothers you, try to forget about it. The pressure that someone might break into the room during intercourse affects your subconcious. Basically you need to have a daily exercise program for an oxigen rich blood, because collagen levels build up in lethargic people. Mostly this problem can be fixed by a change in lifestyle and stress relief. Don't worry too much because this is a problem most men have, but don't really talk about it. I'm 18 and in the same problem as many of you.