I have found glass palettes to be the best palette to mix paint on, whether it is oil or acrylics. Every other palette I have used, be it wooden or plastic, always ends up caked with endless layers of paint. It is impossible to get this dried paint off and it becomes hard to mix colours if you like mixing on a clean and smooth palette.
Glass palettes are fantastic because you can scrape off the dried paint and get back to the original clean palette no matter how old the paint is. Use a single edged razor blade to scrape the paint off.
You can use a big plate of glass on a table, or you can use a smaller portable glass palette. One thing to be careful of is using non-safety glass for a portable palette. I slipped once and dropped my palette which cracked and put a hole in a canvas – very dangerous!
Luckily Dick Blick sell the safety-glass Amaco Paragona Glass Artist Palette for $20 USD. I bought the oval shaped palette a while ago and have been extremely happy with it. It is so good not having to stress about cleaning the palette after a nights painting!