In addition to the great advice above:

It helps to do all your research on the guitar(s) you want. Know everything about it, pros and cons. That way you're prepared. However, when you go in, ask the salesguy everything about the guitar. That'll help you to see if the guy is worth dealing with. If he doesn't know anything about it, then he's not your man. Even if he's an expert, by being well informed, you can always throw something in like "yeah, but this guitar doesn't come with the active pickups like the 2006 JK4512XM". That will sort of level the playing field and he'll know that you're not messing around.

Also, you need to be willing to walk out if they're not willing to budge on the price. Even if it's just for the sake of walking out and coming back in a week and running the same game on a different salesman and getting the same response, at least you can be settled in your own mind that at least you tried.