Probably the best negotiation tool is to be sure you have your car priced correctly. I would go to one of the auto trade web sites, such as Edmunds.com or kbb.com (Kelly Blue Book) and go through the process of pricing out your car for private sale. Be honest in your evaluation of the car's condition, and be sure to include optional equipment in the pricing checklist. The number you get will be the price you should advertise. I would allow for around a five percent negotiation factor, but don't add that 5% to your asking price, because the potential buyer can determine that the same way you did. The price you should be willing to accept will fall between the "private sale" asking price and the average retail price of the car.
Also -- don't try to use silly price "kickers" like claiming that a high-mileage vehicle has only "highway" mileage. None of the auto pricing guides make any distinctions as to how the mileage was accumulated on the car. The only thing that matters is the number that is displayed on the odometer. For example, my Corolla is now over 4 years old, but has less than 45,000 miles on it -- that 's mostly short trip, stop-and-go city driving. However, my car is in great condition, and I am not going to accept one penny less than it's true private sale value were I to sell it that way. And if it had 30,000 more miles racked up at highway cruising speeds, I wouldn't be able to ask for, or get, any more than the pricing guide would allow for an '03 Corolla LE in "Good" conditon with 75,000 miles on it.
The best way to maximize the amount you get for your car is to make sure it is immaculately clean, everything on it is working properly, and all maintenance items are up to date. Make sure the tires match all the way around, and have good tread. Depending on the age of the car, minor scrapes and small dings are to be expected, very few used cars can truly rise above the "Average" condition level as described on the auto trading web sites. So, don't let a small scratch or a dent somewhere deter you from asking for what the car is truly worth. On the other hand, don't claim the car is in "Excellent" condition unless it truly does look like it is showroom new.
The reasonable answer to "how low can you go" should always be "how bad do you want to sell the car?" If you have no motivation to sell for more than 5% less than the average private sale price as determined above, don't go any lower than that. But don't say that over the phone. Have the buyer actually examine and test drive the car, then ask him if he is ready to buy it today. If he says "Yes, but I need you to come off your asking price," counter that with "Do you have cash?" If he does, then you can start to come down in small steps until you reach that 5% off level. Whether you go below that, and by how much, will then be determined by how badly you want to sell.
BTW -- I've only privately sold two cars in my life, both times while stationed overseas and selling to fellow servicemen. In both cases I get my full asking price by being realistic, making sure the car was worth the price, and by not appearing anxious to "unload" the car. If you have something someone wants, and your price is fair, you'll get it.
Last but never least -- always insist on cash. NEVER give "credit" or "payment plans" or any other sort of financing deal. If the buyer needs a loan to buy the car, let him get it from his/her bank. You are NOT a bank. Do not give the buyer the keys and signed title/registration papers until you have 100% of the agreed price in hand -- either in cash or a cashier's check. And, in the case of the cashier's check, don't do the final deal at home. Take him, the car, and the check to YOUR bank and present the check for deposit into your account. Once it is accepted, THEN you turn over the keys and papers, preferably in the presence of a bank officer. An honest buyer should be more than happy to do this. If not, get a buyer who is.