I'm running a Dogs-based game (nonstandard setting) that has lots of high-stakes negotiation in it.  I'm using a negotiation mechanic I call Sweetening The Pot.  Here's how it works:

When stating the stakes of the conflict, the player states his opening offer.  If the offer is generous, the offering side receives 1 to 3 extra d6 (depending on how generous).  If the offer is low, the offered side recieves 1 to 3 extra d6.  Either side can gain additional d6 by upping their offer (or d4 if their additional offer is insulting or niggardly).  If and when the conflict is escalated past Social, the side which has received fewer offer dice gets compensation dice -- one compensation d6 per 2 dice fewer, or one d4 per die fewer.  I suggest using pennies or other markers to keep track of which side has recieved more offer dice. 

The key with this is, as with the rest of Dogs,  to remember that you don't just run out and use it because you can.  You only use it when the stakes of the negotiation warrant conflict resolution.  I've only used this for one session, so it's really hard to tell how it works just yet.  But it wasn't blatantly horrible. 

Lee

Who decides whether an offer is generous or stingy? That seems like it would make a big difference in how this plays out.

Good question.  Whoever is receiving the offer makes that call.