Don't forget about your trade ins value, this fifth step of the car buying guide is often overlooked and can cost you thousands!
You can get a great price on the car you're planning to buy and possibly even buy it a $1 over invoice, but still let the dealership profit thousands of dollars more than they could have, because they "low balled" you on your trade.
Low balling is a term used to describe a dealership showing you $7,000 for your $10,000 car. Instant $3,000 profit to them if you've got no clue what your trade in is worth.
Two of the largest sites for figuring used car values are Kelley Blue Book (western states) and NADA (most others).
Take a look at whichever one is used most in your area to get a feel for your trades value. Keep in mind that these are simply guides and their used car values are not set in stone, but they'll give you a good starting point.