I've got a friend who was a longtime cop, and wound up a captain in Miami. He said that if the cops really want to search your car, they can arrest you on some charge or other, which leads to impounding the car, which makes a search legal.
However, as I understand it, there has to be a
reason to arrest you that would stand up in court, and a simple refusal to allow a search is not a reason. Also, the fact that you steadfastly refused permission for a search is huge, if anything untoward is found.
There have been some Youtube videos on this subject that ring true. One is done by the otherwise-despicable ACLU, but hey, even a broken clock is right once in a while. They provide a very narrow set of things you should say, that, if you stick rigidly to the script, should work. There are several videos clustered around this one:
http://www.youtube.com/watch?v=Glpy3f-tEfY&NR=1Thank God for the Constitution.