This is a common problem, which to be honest I have no solution to other than paint quicker!!! If you need to mix a colour and keep it for more than a week or two I suggest you change palettes. Sorry SAA you didn't hear this! Try an old ceramic plate, mix a small volume of retarder (slow drying gel) into your paint. When you have finished painting, then place some cling film over the plate, sealing it. This will help to perserve your colour for longer. Store away from heat and sunlight. When you return to paint just simply peal off cling film, break skin over the paint surface and your away with the same colour you mixed previously. Once you completed your painting and want to wash your palette, peel off cling film and leave to dry for 24 hours or so. Once paint is dry it can be immersed into hot water and the painting will easily peel away from the palette. I never recommend putting anything like milton solution to a stay wet palette, especially if it comes in contact with the paint at all or possibly the surface you will be working on. It could corrode the palette, paint or surface you are working on and completely ruin your painting long term. Hope I have been of some help.