Go in expecting to leave. My dad starts every convo with, "Just so you know, we're not buying today." He ALWAYS takes a night to sleep on it and tells them so. Never in my life seen him miss a deal he wants.
Also, go in with the attitude that "THIS" isn't the only one. If you find something you REALLY want, understand that another is just around the corner.
Don't let the salesman do his job. He'll try to persuade you to buy whatever he wants. Stay firm and as said before, don't "show fear".
Walk. The greatest deals I have gotten for myself and others come when we can't reach an agreement so I walk. My brother got his GC, that was listed at $13,900 and needed tires, for $10K WITH new tires. We walked and that night when we got home had a voicemail saying they'd make the deal. I simply told them he was preapproved for $10K and stuck to it (although he was app for $12K). They say things like, "WE can't do that. The vehicle is worth more." My rebuttle? "I'm sure it is, that's not in question. The fact is he has $10K to spend and doesn't like any other car on your lot. If you can't make the deal we understand and thank you for your time."
Just make them play YOUR game. Keep the ball in your court. Let them know you aren't buying right then and there and that you WILL be visiting other lots and taking a day to think it over.