Here are a few things I've used to help manage potlucks.
1 - This is actually something my mom did. It's the one thing I remember her doing, and now I know why. We had a, not-too-large pressure cooker that she used to carry anything that had a lot of liquid in it. The seal kept it from spilling in the car. I don't have one, and try to avoid making anything with a bunch of liquid in it to start with, but my most recently purchased slow cooker has a rubber gasket and locking clamps that will do the same thing.
2 - Clean, gallon jugs, especially if they have screw on lids, are much better than most pitchers for carrying tea or other drinks.
3 - I use a heavy duty laundry basket to carry things in when I have a lot that needs to go. Laying a towel on the bottom can help if the dish is hot, but it also helps keep it from sliding around in the basket. Also, if your dish is hot, the towel can be folded over the top and it will keep it warm.
4 - If you have something that has been cooked in a 9"x13" casserole dish, you can put a cookie cooling rack on top, and stack other things on top of it. If the dish is still very warm, just throw an extra towel on top before putting anything else in the basket. It's a great place to put bread, if you happen to be taking that to the dinner.
5 - Non-slip shelf liner can be rolled out in the trunk, or other storage area, to help keep things from sliding around.
6 - Find some way of labeling your dishes and utensils. You can write directly on plastic containers with a sharpie marker, you can use a label maker, if you have one, or you could even write your name on a piece of masking tape and put that on your items. Whatever method you use, it really helps everyone keep up with what goes with who - or helps the host(ess) know who's spoon was left at their house.
7 - Take some aluminum foil, plastic wrap, and/or ziploc bags with you. You never know what you may need to wrap up. It's almost a necessity if you're at a community center, or other, similar place, but it's also a good idea if you'll be at a friend or family members home. Many family dinners are held at my house, and I know from experience, the supply of these things can be exhausted in a hurry. Most people, myself included, don't really mind, but having to replace all your wraps and bags at once can be pretty pricey. It would just be a thoughtful way to help out whoever is hosting the get-together.
8 - If you, or other members of the family have kids, make one of your dishes macaroni and cheese. Then, no matter what anyone else brings (like the eggplant casserole "Aunt Bessie" constantly shows up with), you know there'll be something there for the kids to eat. Trust me, if you do this enough, you'll become one of the kids' favorite relatives. :D
9 - Crock pots are great for taking food to functions where you'll have to wait awhile to eat. They can be plugged in and left on warm until it's time for dinner to be served. Most of the families at our church do that for the potluck meal we have the first Sunday of every month.
My final tip is for when you need to take food to someone for reasons other than a get-together. Maybe there was a death in the family, a new baby was born, or someone just had surgery, or moved. It's wonderful to be able to bless others in this way, but sometimes people are left with a bunch of dishes they need to return. I like to use aluminum pans, or the reusable Glad pans for the regular food, and bakery boxes for cakes. There'll be nothing to clean up, unless they want to reuse the pan, and they don't have to worry about getting my dishes back to me.
I hope you found a useful tip in there somewhere, and if you do find yourself with a spill in the car, carpet cleaner works wonders.