There are a few solutions:
1. Say no.
2. Ask for a reciprocal concession.
3. Think of a win-win way to get them what they want.
Read "Bargaining for Advantage" for more: http://www.amazon.com/Bargaining-Advantage-Negotiation-Strategies-Reasonable/dp/0143036971/ref=pd_bbs_sr_1/103-5417622-0009467?ie=UTF8&s=books&qid=1180628056&sr=8-1
You can't trust people who treat every interaction as a one-time play of the Prisoner's dilemma. A long negotiation is playing the game many times. If the other guy plays "screw" repeatedly your only option is to play it back or quit playing. (An aside: I once worked for a CEO who was an ex-DEA prosecutor. His entire experience was dealing with criminals once and this mapped very poorly to dealing with employees. I quit.)