One rule I've found very helpful is to never give a concession without getting something in return--or at least asking for something back. If you don't, you'll end up negotiating with yourself. Only in this situation, the buyer keeps the process going by making more and more demands. You can control this too by asking for something in return. For example: "If I extend your warranty, will you buy the extra large assortment?" It's a not-so-subtle reminder that negotiation is a two-way street. It's also a graceful way to avoid saying no.
Excerpted from Creative Selling: Boost your B2B sales