I need advice on how to successfully haggle for a used car

I hate haggling. I'm too polite, I don't like to make people mad & disappointed, I just usually accept whatever I can get.

But I need a new car (well, used car anyway), because my current one is a lemon. However, I'm exactly the wealthiest person & I need advice on how to haggle with cars.

Some background info: My preference is a Toyota Camry (or a similar larger 4 door car) that isn't more than 5 years old with no more than 80k miles. I ideally don't want to pay more than $250 a month for 5 years. I won't finance through the dealership, I'll be getting a loan from my credit union (The interest rate for me is about 3.5%).

Besides shopping around to different dealers & getting prices & what advice can you guys give me on successfully haggling for a used car? (Preferably without getting mad at the salesperson & walking out of the dealership angry)