Hmmm. I don't know man. I just picked it up today so I'm only a rank 8, but I've only had one game where I went negative.
Play like me, run around everywhere and shoot everyone... that's what I do. Oh! and blow people up with grenades that's fun to. Bashing people with your gun is fun too. Me and 3 other dudes all got jammed in a room together and I bashed the shit out of all 3! Good times!
Do you know how the armor/stealth stuff works? I haven't figured it out yet... I was rank 3 before I even knew how to do them.