All you have to do is say this if they start asking too many questions "Officer, I know you are just doing your job but I would like to exercise my right to remain silent and I do not consent to a search." Also do things like roll up all your windows except the driver window and have it cracked about 6" so they can't pull the "in plain view" thing and also odors won't escape as much.

SWIM (i'll only use SWIM in this one post) used to drive a couple pounds of stuff at a time for a friend and I got questioned a couple times at road blocks checking for drunk drivers and such. If they start asking questions at a road block, say that you are in a hurry to get to your friends house and say "Officer, am I being detained or am I free to go?" Be polite and make sure those roaches are out of sight!