Payment- how do you deal with it?

I offer different prices based on whether it's before, after or 50/50.

I prefer getting payed in advance not so much because I suspect they won't pay otherwise but because you have money you can invest for gear immediately.

I always tell the band I'll keep mixing their songs until we (the band, the audience and myself) are satisfied with the end product or until their deadline comes. There have been cases when I've had to spend way too much time mixing considering how much they're paying me, but I think it's worth it.
 
I've never been too stringent in this area, but I have a feeling that will change fairly soon.

50% up front and 50% upon completion sounds fair to me. 100% up front is ideal as it jumps over the need for watermarking and all that junk, but one wonders how many clients are that dedicated/organized with their finances.
 
cover your costs + 10% up front - remainder is paid when they hear the final mix in an environment where they can't steal cd's etc - ie: your DAW - they pay the remainder if happy with the mix they hear - it gets the final post production bounce and burned along with the session files