Just some random thoughts from personal experience...

If you're giving a handjob, just let the cum go wherever it goes. Don't worry about it - just let it happen. Neither of you will enjoy it if you're worrying about what's going to happen to the stuff. (If you want to avoid getting mess on sheets, clothes, wherever, throw down a towel, sheet, T-shirt or something before you get to it - not halfway through!)

A really erotic thing is to ask whether the guy would like to come on your breasts or on any other part of you.

With a blowjob, I think most men love it if the girl swallows. It's a really amazing feeling. But it's up to you. The man should let you know when he's coming so you can do what you want. You'll probably know when he's nearly there. If you feel like swallowing, don't tense up, don't panic, just try and relax. Just swallow it like you normally swallow stuff. Be gentle on his penis as it will be sensitive. A dirty "bonus" is to "clean up" any spillage with your tongue.

If you don't want to swallow, just switch to a hand-job but maybe keep your head close by (and out of the way) so it doesnt seem like you're keeping well clear!

General tips:
Make sure you both enjoy the moment of coming. Enjoy watching and feeling it. And enjoy the warmth of it.
Don't clean up right away, let the orgasm pass, and enjoy the moment.
Ideally, have a warm damp cloth handy to wipe up afterwards. But Kleenex is fine. Either you can clean yourselves or one of you can the other. Some people don't bother to clean up - makes sex dirtier.. just enjoy it.