My boyfriend works at a car dealer, and there are two things that will help you get a better finance rate.
The first is coming in with your own credit score, as others have suggested. However, realize that having your credit score (good or bad) in hand won't mean anything if you don't know what rate you should be getting. I bought a car two months before my boyfriend went to go work in the business, and seriously... the finance guy looked up into space and pulled a number out of the air. I mean, he could have at least PRETENDED to look it up on his computer!!
So, the second you can do is having a (low) competitive offer from a credit union/bank/etc IN HAND. The dealership makes money on the financing, and if you have an offer in hand, you can see if you can negotiate to get them to beat or match the offer. If not, go with the CU.
Bottom line: you can negotiate the price of the car, the amount of your trade-in, and your rate of financing. Take your time, shop the prices they quote you, shop the rates they quote you, bring in evidence of everything.