I realize that not everyone is as big a space cadet as I am, but some things I learned from the previous swap.
Water the plants you are bringing the day before the swap. Watering the day of tends to create ugly messes in the car. I have a messy vehicle in the first place, but scraping caked on mud off the upholstery is so not fun.
Label the plants using something water safe. Conversely, don't label them until the plants have been watered.
If you're going to put pre-arranged stuff into separate cardboard boxes, make sure that you do it after all this stuff has dried off. Keep the boxes out of the rain, and don't water after you've put stuff into their respective boxes.
When possible, bring something that describes and has pictures of the plants you have. It is much easier to sucker people if you can show them pretty pictures.
Do not leave non-gardening spouses alone in the midst of gardeners. Makes them even more dazed and confused than they were in the first place.
The last is a request from my DH. Pretty please with sugar on top, don't talk plants, especially anything involving latin names with him. He'd really rather not know anything more than 'That one has pretty blue flowers'.