Looking up foreign expletives I've found that bodily functions appear to transcend all culture boundaries, but since I don't feel comfortable with my characters swearing (and they wouldn't, on the whole) that leaves me with "crap", "bugger"(**), and lots of goofy sounding expletives like "blimey".
I can get by with these, but it would be nice to have a wider range of exclamations, and once I have properly decided on religion it would be nice to subtley reference it. A lot of fantasy has things like "By Thor's hammer!" but they feel rather forced. I'm curious to know what people actually say, though I'm aware that this sort of thing (like folk sayings) usually sounds dumb when taken out of context.
I know I could just make some up, but a little cross-cultural research never hurt :)
(*)I'm still a little shaky on what my characters all believe, but while some of them are monotheistic none of them believe in a hell, exactly. I think :)
(**)And while this is a fairly mild epithet in Australia I still feel a bit uncomfortable with it. Yeah, I need to stop being a such a wuss.
EDIT: thanks to everyone for all your useful comments! Something I didn't make clear is that partly from personal interest (since as you pointed out, its pretty easy to extrapolate curses for a made up culture), what I'm really interested in is a cross section of authentic blasphemy from different cultures.
But some of you have answered this question despite the opacity of my request, and you have all given me plenty of food for thought for my actual story, so thanks again! I only discovered this community a few days ago, you guys rock :)