There was an article in the Irish time (I think) recently about negotiation over the phone when buying a car. The basis of it was to identify 5 or 6 cars online of the same make/model/year that you are interested in.
You then ring each garage in turn, and outline exactly your situation - e.g. "I am going out this weekend to buy a 2006 (name/type of car etc) with cash, no trade in. I have identified 6 cars that fit my needs, one of which is yours. What is your absolute best price on that car."
If they give you a price, you tell them if it is already been beaten or roughly how good it is in relation to the other quotes, even telling them they are your first/second call etc - and you give them your details and instructions to call you back if they are going to do any better. A bit like buying insurance really.
According to the article, some dropped significantly, orthers not much. Bottom line is you avoid calling round garages, and hassle of face to face negotiation. Probably works better in Uk type market, but worth a try here no doubt, as long as you know what you are loking for and can find a few examples.