› firstname.lastname@example.org 
I'm having the best sex of my life, but when I'm having a good time which is often my PC muscles have minds of their own and they get enthusiastic. I know I've got strong PC muscles because the last time I went to Doc Stirrup she told me to squeeze and then said, "Whoa." The end result is that I inflicted one doozy of a bruise on my poor guy's junk.
He's being a sport about it and says he doesn't mind, but I know it hurts him afterwards and I'd rather not strangle my man.
I think we'd all rather you not cause permanent damage, physical or psychic, to your sweet baboo's manhood (also either physical or psychic, come to think of it), and I do think I can help, although I understand that you are a woman to be reckoned with and he probably shouldn't take anything for granted. (Note: I know the writer slightly, and nobody would mistake her for anything less than a force of nature, although obviously I had no idea just how much of a force. Bruising! Really.)
Now here's the thing: the whole deal with yer basic dentata muscles is that they do operate via conscious control, so even though you'd rather be all transported and let your eyes roll back in your head and all that, you'll need to think, really think, about relaxing those muscles while you're at it, exactly the way those with less-toned bits have to concentrate on contracting them. In fact, perhaps it's best to look at this entire problem backward, if you will.
While your (boyfriend's) problem is not unheard of one can, for instance, rapidly lose all feeling in one's hand after inserting it up to the wrist in the terrifyingly well-toned interior of a Kegel-exercise enthusiast the opposite complaint is far more common. When a woman can't feel much upon intromission, or her male partner finds himself diligently thrusting away but has to keep reminding himself that he isn't just pumping blindly into thin air, then it's time for some Kegeling and some applied mindfulness. I suggest that you practice not contracting your pelvic muscles when excited, either with his help (warning: this exercise is not particularly erotic), or alone, or both ways. Women trying to get their muscles under conscious control can buy something such as a "Kegelsizer" or "vaginal barbell," even. These are rather lovely, smooth, heavy devices of stainless steel or similar, and one practices holding onto the larger, more bulbous end and progresses to the smaller, at which point one may also be able to project ping-pong balls across the barroom or smoke a cigarette in an unexpected manner. (But of course you're not interested in such circus tricks. You're not, right?)
I am quite sure that you could employ such exercises in the pursuit of less instead of more, since it's less reflexive clenching you're after, not less muscle. Just do be careful not to accidentally ultratone yourself. You could break something.
There are also, of course, tips and tricks for genital-size-discordant couples that could be brought into play here in reverse. Women who want more friction for themselves and/or their partners keep their legs close together, so do the opposite. The famous but not-for-amateurs modified missionary position where the woman lies supine and the man straddles her legs, keeping them clamped between his manly thighs lest they dare to make a break for it, is another obvious no-no. The one with your feet up around his ears while he clutches your hips? Don't do that. Also, all those tricks for better alignment (hip-tilt pillows and whatnot) are meant for G-spot (internal clitoral) stimulation, but that is accomplished partly by just making things tighter in there, so they're contraindicated too. I'd also throw in whatever you yourselves do in pursuit of greater sensation, since in intercourse sensation is linked to tightness, which is linked to friction, and quit doing (briefly, we hope) whatever you were doing when you caused the bruising. Remember, we're in Bizarro World here, so whatever feels especially intense is on the "quit it" list, at least until you get those Supergirl muscles under control. And in the interest of equal time for opposing cartoons, stop eating spinach.
Now, let's consider lube. Lube is tricky, since it actually decreases friction yet improves sexual sensation, making a lie of what I said above about friction, but never mind that. Yes, I tell people who aren't feeling enough to try more lube, and yes, I tell people who are feeling too much to try more lube. What the heck, it's cheap.
Andrea is home with the kids and going stir-crazy. Write her a letter! Ask her a question! Send her your tedious e-mail forwards! On second thought, don't do that. Just ask her a question.