At the beginning of the summer I designated a different colored bag for each child. In it, I packed summer essentials: t-shirts, shorts, a long pair of pants, a sweatshirt, swimsuit, sunscreen, toothbrush, toothpaste, beach towel, socks, PJs, etc. Stores like Target, where cute clothes can be found at great prices are helpful with this system! When it was time to go, we picked up the bags and off we went. Here's the key -- they kids can't touch what is in the bag at home (there is no guarantee it will make it back in). At the end of each trip, I'd take the bags into the laundry room, and wash the contents of the bags one at a time. I then repack right from the laundry basket back into the bag. Now it's ready for the next outing!!
Maybe you have already thought of doing this, or something like it. Summer doesn't last very long around here, so anything I can do to spend less time packing and more time enjoying it is a good plan for me!