I've found it really helps if you stick with a theme during crops. For instance, at one I scrapped all my Christmas pics and at another I worked on my wedding album. It really cut down on the amount of stuff I had to bring because I knew I wouldn't be scrapping little boy pics. I also skip bringing things like stickles and embossing powder - that's just too messy for a crop and there's nowhere for things to dry.
I have an all-weekend crop coming up at the end of the month and I've got to get cracking on what to bring!