I once did a two-day workshop on negotiation techniques, which covered not only methods but also helped me to become more comfortable dealing with the psychological stress that's induced in many negotiation situations. So far I'd say the course (which was free as I won it as a prize in a business plan competition) is directly responsible for at least 50-100.000 € of additional revenue that I made in the last years, s…
For instance, my favorite tactic when someone is pushing hard to sell me something or negotiate is the "Appeal to the authority" (i.e. "Let me first talk about it to my wife (or co-founder) about it."
Another popular negotiation tactic is to get asked between option A or option B, where in fact you could simply respond with "option C is more what I'm looking for".