Internet shopping my man. You email every dealer with in range of you, tell them what you want and tell them to give you your best price. Go with the winner, however, once you've picked it, print the stuff out, because by the time you get there they will have "sold" that car or change the price for whatever damn reason.
Look, negotiate the price first, don't tell them you need financing, then negotiate your trade if any, finally the financing. Best if you have financing already lined up. Like through your bank or credit union. Then go down there, don't tell the guy what your rate is, ask him for his best rate, his is better or worse, doesn't matter, don't tell him what your other financing rate is. He doesn't need to know that, if he wanted to beat it he would have already.
That's in a nutshell, never be afraid to walk away from a deal, I've made salesmen, and in particular sales managers, red in the face because I was well prepared when I got there and explained to them I wasn't paying that damn price, or better yet tell them you've got a better rate or price at a different dealership, that really pisses them off. It's all about the thrill of the hunt.