Read books about how to negotiate. You can stand on the backs of giants. I’d start with “How to win friends and influence people” by Dale Carnegie, as it’s a good guide on how to just deal with people.
Tinkering with technology is my hobby, so I spend a lot of time on secondary markets (like Facebook Marketplace, OfferUp, Craigslist, eBay, etc.) I always research the original price of any item I’m interested in, any current discounts being offered by retailers (like Amazon, etc.), and what the average pre-owned price appears to be across platforms. Then I offer some amount less, and expect a counter. I equate negotiating with fishing. I know that, with patience, I will eventually find a motivated seller. So I try to never enter any specific negotiation needing to make a deal. Obviously the rarity of an item and your desire to obtain it are big factors that will influence the price which you are willing to pay, but knowing the item’s value and being unafraid to make a reasonable-but-low first offer has proven successful for me. Also be courteous. I have had better success providing context and asking questions about the item then merely opening with a low effort bid, and have even had several sellers return to the negotiation hours, days, or weeks later because I invite them to if we are unable to reach an initial agreement.
I use this all the time. At some point in my life I just stopped being uncomfortable with silence, but it drives most people mad.
And it’s not just useful in terms of having an “opponent” stew in some social discomfort.
Use it to take a moment to think. Use it to give the other party time to think. Use it to let your most recent words hang in the air and gain more weight. Etc.
Everyone’s a comedian so here’s some actual advice.
TLDR: be polite but firm, know what you’re asking for.
Lots of activism and lobbyist groups have the concept of an “ask”, or what you’re attempting to get out of the negotiation. Have a solid, easily understandable ask that is generous enough to withstand a bit of pushback from the other party, who will almost certainly try to negotiate the other direction. Also, when you deliver a statement, question, or ultimatum, wait for the other person to respond to it, even if there’s a couple seconds of silence in between. Conventional wisdom says the one who speaks first in these moments is displaying insecurity and is less sure of their ask or position in the negotiation.
Good advice. It's also good to have clear boundaries set up. The ideal. The what you expect, and the non negotiable. Start with the ideal. Expecting to get pushed towards what you expect. Never budge on the nonnegotiable.