“Capitalize on differences… smile when you are negotiating and you have differences with your counterpart.” You will strike the best deals when you identify and quantify the differences in interests. Jeffrey Sanchez-Burks said it best in this 90 second video.
When you negotiate like a professional you use a process that includes identifying a wish list for your counter part. Professionals note and quantify these items. Ask a lot of questions, listen carefully, and concentrate on what the other party is trying to accomplish. You will uncover their wish list items. As the negotiation develops you have this list to choose from and may offer concessions. You will strike the best deal when you identify and quantify the items on their wish list. You can offer trades for items that cost you little [or nothing] and mean a great deal to your counter part.
If the idea of creating a wish list is something new for you. Consider using something like this negotiation roadmap to document and track your next negotiation.