Using a sharp blade on the miter saw helps a ton (I learned that the hard way on my bathroom remodel). But is it really worth stressing over? Most gaps get covered with paint and caulk anyway. I sometimes clamp the piece down better instead of just a backer board, which stops the wood from moving and splitting.
Read on a woodworking site that a super sharp blade and cutting really slow helps a lot with the splitting... The slower feed puts less stress on the wood fibers. Some guys even say to put a little piece of masking tape over the cut line. A good clamp setup is key too, not just a backer board.