I just use old pot lids as mixing palettes .. when they are completely filled up with dry paint I throw em out and take a fresh one that lasts me about 4 months.
In the meantime I was using cheap plastic palettes from artist stores, CD cases, and stuff. Lately I use a bathroom tile, it has a very smooth and hard surface, easy to clean. I clean it after every painting, as I don't like that I have to look for the current paint I'm using, what happened to me while using CD cases for instance, and the whole surface was already covered in paint. So basically it disturbes me when I mix a paint and underneath it's not white.