Never consent! Always refuse any type of search. Even if it may seem odd, you preserve your right to contest the matter if the search is not legitimate. However, never resist if a police officer insists on searching. If he is in violation of your 4th amendment right against unreasonable searches and seizures, a Judge should suppress the evidence obtained in court.
If you do verbally give your consent, you can also revoke this consent – BUT IT MUST BE CLEAR AND DIRECT. You cannot waiver. You must clearly tell the officer you would like to revoke the consent given for permission to search. If he continues, let your criminal defense attorney contest the matter in court.