While I was at university, I was a staff member for our newspaper. I reviewed computer games - I even got free ones sometimes, but they were all shit. But I enjoyed it - because I like games, and I like writing, and I was a close friend of the editor, so I used to pop into the office to chat to him once in a while. I was, however, a little surprised that, in the first issue he edited, he included a full-colour spread on what your orgasm face meant. (This was, it turns out, a cunning advertising placement by Trojan, and in retaliation I've always bought Durex.) I was a little interested - and bored - so I read it.
According to the article - and, surprisingly, this tallies with my personal experience - one of the best things your partner can do (assuming they are a girl, and I've just realised how sexist this article was) in bed is to giggle, or at the very least break into a smile, during orgasm. I like it when that happens.
Not least because people - the obvious exceptions being myself and, for the record, Lord Voldemort - look better when they smile. I've never had sex with someone I consider unattractive, of course, but there's something about the act of a broad grin unfurling on somebody's face that really brings out their beauty in multi-layered ways. My girlfriend looks utterly radiant when she smiles, and there's a definite cheeky grin there post-orgasm (during orgasm, I'm usually too busy doing other things, like continuing to do whatever it was I was doing before because I'm unstoppable, or something like that).
There's also the point of looking at the other person during sex (missionary, or astride, are good positions for this, and also coincidentally my favourites - it's good to see who you're making love to), and seeing a smile on their face, assuming they're not a girl in a Hentai game and will accordingly look like they're in extreme pain. You can moan through your mouth, but I think it's nigh on impossible to have passionate sex and keep a straight, serious face all the way through, and while drawing breath (which you have to do at some point - airway, breathing, circulation) you may well smile without knowing it. Fun, eh?
And then there's tickling. Now, I'm probably the most ticklish person in the whole world, but even if you're not, during orgasm all your nerves increase in sensitivity, and if you touch, well, anywhere (but let's say a specific place) immediately following its subsidence, you can make your partner laugh - usually accompanied by a squirm and a remonstration, but it's a laugh, and all in good fun! You can probably even kick off an aftershock that way, which I like doing, too.
So, yeah, an open mouth and a loud moan - well, that's fine, but it's more likely to be faked (although, in all likelihood, probably); a giggle, or even a grin - that's apparently the best reaction to get. Maybe it sounds odd, but it's certainly worthwhile. The best thing is, you don't need to try - it comes automatically...
...except if they are laughing at you. I hadn't considered that.