The secret to buying a car, new or used, is to be completely educated about what your buying. Pick your car on the internet, know the features, mileage, etc. Go to kbb.com to find the blue book price or dealer invoice, talk to your bank first and know the interest rates; that way the salesman has nothing over you and you can just make the offer and walk away.
I haven't had to haggle the last 4 cars I bought, I just tell the dealer what I'm willing to give for a car. 3 of the 4 times they took the deal on the spot, the 1 other time they called 2 days later and accepted my offer.