I saw Mark mention vacation time as something you could potentially negotiate.
I have only done this once. I asked the hiring VP if he would match my current vacation after I had completed a year with them. I told him I would have to work extra hours to get up to learn their company and products starting out, and since I didn't have a track record with him I wouldn't expect him to do this starting my first day.
The hiring VP thought about for a minute and said "HR is going to hate me for this... but that would be fine".
How has everyone else approached this? Is there a better way?