A person I know was once stopped by law enforcement and browbeat into giving permission for their vehicle to be searched.  Once the consent was given, the officers literally tore apart the truck.  When the search was over and nothing was found, the person was left sitting beside the road with the seat sprawled there beside them and all the contents of the vehicle strewn about.  The person had to return everything back into the vehicle themselves and reattaching the seat meant an expensive shop vist.

Know your rights says Neill Franklin, a 33-year law enforcement veteran and former training commander with the Maryland State Police and Baltimore Police Department.

What few people understand, but police know all too well, is that your constitutional rights only apply if you understand and assert them. Unless they have strong evidence (i.e. probable cause) police need your permission to search your belongings or enter your home. The instant you grant them permission to invade your privacy, many of your legal protections go out the window and you’re left on the hook for anything illegal the police find, as well as any damage they cause in the process.

Of course, even if you know your basic rights, police officers are trained to shake your confidence. If you refuse a search, I might respond by threatening to call in a drug-sniffing dog and sternly reminding you that things will go much easier if you cooperate. Creating a sense of hopelessness for the suspect enables us to break down their defenses and gain compliance.

See here for the ten rules in dealing with police.