A couple of tips I've learned:
-don't use a paint brush. My boy didn't want to get too dirty, so he refused on using his fingers. The end result was a gooey brush that didn't get all clean and hardened.
-do several different recipes to find one you like. I wasn't too impressed with my recipe, so I won't share it here.
-the paint settles, and the thicker stuff goes to the bottom. I drained the runnier stuff after a night settling, and I haven't tried them yet.
-because of the flour-and-water mixture, it's technically a glue, and it's really hard to get off surfaces once its dry (like the paintbrush).
Now that I drained off the liquid, and I'm only left with the gooey stuff at the bottom of each jar, I think it might do well in the bathtub...