Scott Morgan explains:
It's always possible that police might search you anyway when you refuse to give consent, but that's no reason to say "yes" to the search. Basically, if there's any chance of evidence being found, agreeing to a search is like committing legal suicide, because it kills your case before you even get to court. If you refuse a search, however, the officer will have to prove in court that there was probable cause to do a warrantless search. This will give your lawyer a good chance to win your case, but this only works if you said "no" to the search.
The Dish 