1. Keep your paint in ice cube trays. No, don't keep them in the freezer. Ice cube trays make the perfect paint holders! I use water color paint and I like to alternate spots so that the different colors aren't so close together.
The spaces are deep so it is hard for the students to mix the colors (unless they're trying to). They are also very easy to stack. When the students are done, I just stack the trays on top of each other. When I want to use them again, I just pass them out. The students add a little water and POOF the paint comes back to life!
2. Ask each student to bring in an old shirt. You can do this by sending home a separate note or by writing it on your class' newsletter. The bigger the shirt the better. I always tell my students to bring in one of their parent's shirts or older brother or sister's shirt (these cover more of their clothes). Make sure they (or you) write their name in their shirt. I keep them in my room and whenever we paint they put their shirt on. This keeps their clothes SO much cleaner. I have yet to get an angry note from a parent about their child's dirty clothes. I keep the shirts until the end of the year, wash them, and then send them home. Normally I ask for these at the beginning of the year but you can do it any time.