Negotiating a Better Salary

I wonder how common it is to negotiate a better salary? I wonder because I always do but it seems to me that most people don't. I feel like that is just like leaving money on the table.

My girlfriend recently got a job offer. She was happy to take it for what they offered, but I convinced her to ask for more; I mean the worse that would happen was that they said no. So after making a counter-offer, they counter-offered back, and she accepted. She ended up getting a little bit more money then they offered, but not quite what she wanted.

I have always done the same thing. I have never taken the first offer that somebody gave me. They always say, "Our first offer is normally our best offer" but that's a load of crap. If that is true, how come each time I've tried to renegotiate I got more money?

Maybe I'm fortunate. Maybe I'm just so super-special that people just want to throw more money at me. I'm curious if people have had any experiences where they asked for more money, and they didn't get at least a little bit more.