I am pretty much the traditionalist when it comes to foundation blocks. I wash first either bleached or unbleached muslin. Works great for string blocks. I have also used a paper that I have pre-measured to the size of block I want and do it like a paper pieced block, removing the paper afterwards. The only thing with that is the possibility of bias cuts and then the block is unstable. So I prefer using muslin.
So many of the other suggestions here are great! I may even begin to save dryers sheets for this purpose.