When I travel by car I make sure that I bring or pack several size kinds of bags both cloth and plastic. If you've been keeping up with me you know that I am an advocate of traveling light. I tend bring a bit extra camera gear when I travel by car but clothes packing tips remain the same. My packing goal is to have everything fit into the trunk of my car. My camera being the exception to that rule but my camera bag usually remains in the trunk when I'm driving.
I take along several shopping bags as they make handy impromptu suitcases if you happen to need them. They also make pretty good laundry bags if you end up doing laundry. I also use the cloth bags for those trips to the store to bring groceries back to the hotel.
I have several size plastic bags from zip lock sandwiches bags and quart size plus a couple of 30 gallon trash bags. I prefer to separate my clean and dirty clothes so I put the dirty clothes into the trash bags.
All of my liquid products go into zip lock plastic bags. If I'm going to be gone for several weeks I get smaller bottles of the products I use and pack those. If I'm going to be gone a few days I just fill some six ounce bottles with my usual stuff and pack those in plastic bags. I usually have hair and body products in separate bags and bundle those into one larger bag. I don't put any liquid products in my suitcase. I prefer to put them in a cloth shopping bag that I put in my trunk.
I also use the quart size bags for protection of my camera if it rains. I cut a hole in the bag and stick the front portion of the lens through. A little gaffers tape on the lens hood and plastic bag and I'm good to go. Not the best solution but it works in a pinch and is much cheaper than those camera raincoats. Living in Arizona I don't need rain gear very often, but just in case I do have zip lock bags, garbage bags and plastic raincoats in my camera bag.
The quart size bags are very good 'raincoats' for protecting my Speedlites and Pocket Wizards around all types of water either made by nature or man.
So when you travel don't forget to take along some extra bags they come in very handy.
Remember to take time to enjoy your trip.