Keep your doors locked and your windows rolled up. (rolling down windows may give them probable cause to search by claiming they "smell" alcohol, weed, etc)

Your sign should read:

I Remain Silent
No Searches
I want My Attorney

Please put any tickets under windshield
I am not required to sign - 318.14(2)
I am not required to hand you my license - 322.15
Thus I am not opening my window
I will comply with clearly stated lawful orders
Check this link for your state:

print the sign, carry in your Glove compartment at all times.