Hey, that's what I'm playing too!
Anywho, here's some tips for demon negotiation early on.
-Don't give demons Macca! Just refuse. Macca's really hard to get early on.
-Cheat; If you get the option and it's on an item (or whatever) you really don't want to give up, may as well attempt it.
Just assume it only works once though. Of course if it fails, expect an attack.
-End Talks; If you choose this, usually it'll get the demon to join you with pretty good success. (It's certainly not guaranteed, but a decent chance nonetheless.)
Also, as mentioned, save often!
It often doesn't take much for things to go wrong quickly.