I second the recommendation to read Getting to Yes. Awesome negotiating book. The big takeaway for me was separating people and positions. Most people get those two entangled, but if you can figure out ways to keep them separate you can often find alternatives that neither of you thought of. The normal, non-optimal negotiations are back-and-forth, meet in the middle kind of stuff.
I like negotiating directly with the seller. For buying investments I do a lot of marketing directly to owners. This allows me to have conversations to learn more about them and they can learn more about me. This face-to-face process has led to many accepted offers at lower prices and with better terms (owner financing, options, leases, etc) than I could have ever gotten long distance. I've even driven 4 hours one time to meet someone in person and present an offer. I still have the triplex in a wonderful location that I bought as a result.
I'll also throw in The 7 Habits of Highly Effective People as a great negotiating book. Habit #4 (Think Win-Win), #5 (Seek First to Understand, Then Be Understood), & #7 (Synergize) have all been incredibly helpful for me in my negotiations.
Hope that helps!