Janus Boye has a great post on negotiating contracts with vendors. I usually try to keep myself out of contract negotiation but occasionally I am asked advice. I agree with everything Janus says and would only add that if you try (and are able) to take advantage of a vendor, it does not bode well for your future relationship. Like I have heard Graham Oakes say, if you don't try to make the deal a win-win, you will probably wind up being on the losing end of the deal. Hopefully, if you have gotten to the negotiation stage, you have found that you can work with vendor and can mutually benefit from doing business together. Work collaboratively to construct a deal that benefits both parties.