Be able to walk away
In any kind of negotiation, your ability to walk away is your strongest tool.
Those who can walk away from the negotiation — legitimately walk away, not just make a show of it — are in the strongest position. Those who are convinced they need to make the deal are in the weakest position.
This is true of negotiating when you're buying a car, closing the sale of your new home, haggling in a foreign flea market, or trying to get a raise.
It's also true of anything in life.
Know that there's almost nothing you can't walk away from.
If you are convinced you need a nice house with a walk-in closet and hardwood floors and a huge kitchen, you now have a weakness. You will give away precious life hours and savings to get it. Someone else who knows that those things aren't absolutely necessary can walk away, and not need to spend so much money (and thus work hours) on that kind of house.
If you are convinced that you need Starbucks grande lattes every day, or an iPhone or iPad, or an SUV or Cooper Mini or BMW … you are in the weak position, because you can't give it up. Someone else might know that those aren't essential to happiness, and can walk away.
If you know that the man who is treating you badly (but who you just know will change someday, because, you know, he loves you) isn't necessary for you to be happy, you can walk away. If you know that you can be happy alone, and that you need no one to make you happy, you can walk away.
If you know that there's almost nothing you can't walk away from, you can save yourself tons of money. Years of time. Mountains of headaches and heartaches. Boatloads of suffering.
You don't need to walk away from everything, but you should know that you can. And when the cost of the deal is too great, too dear … walk away.