I think the best approach is to simply learn how to control yourself during intercourse. (takes lots and lots of practice!
)
Rubbing one out beforehand, especially multiple times just makes the climax when you are with her a little less...well, climactic.
For me, positions are key, I have found the ones that are easiest to control are her on top or reverse cowboy. Doggy style while you stimulate her clit is also a good one. That was my last girlfriends favorite position and it allows for really deep penetration...which is always nice.
Bottom line is that knowing yourself and being receptive to what's going on with her is the key. Pay attention to the pleasure that BOTH of you are feeling and go with it.
Ok, think of Goofy if you must.