If I am doing sound recording, I work out a deal with the producer, director, or whoever hired me. Unless there is some sort of back end, I have a rate and it is paid. I always make sure there is a letter or invoice or email that states the terms with the hiring party agreeing. That's generally enough for small claims court should they back out on payment.
For sound design, I have a standard contract that has some plug in options for pay up front, back end, etc, as well as allowing me to retain rights to any SFX that I create, rights to use any part of the film for my demo reel, and letting them know they do not have the rights to reuse any sound I deliver on any project but that one. I usually have a portion that states I get airfare, hotel, per diem, and a limo to all shows at any festival or premiere. This isn't because I expect to get it (one of these days....), but it means I won't get any push back on retaining rights. Producers, in my experience, always want to talk you down from whatever you want, so I always start with a lot of demands and then remove them until the contract is where I wanted it to start.
I keep everything in paper form with a digital copy archived. This is for my benefit, as well as having it for filing taxes.
I'd rather get the unpleasantness out of the way at the beginning and have a good working relationship than leave on a bad note.