So, fold them! They are paper ya know.
Er, um. If I may make a suggestion, roll
them. Folding them creates creases, creases which may hide navigational aids or hazards. The creases eventually become weak spots and holes in the chart.
You can buy a water-proof, plastic, cylindrical chart case pretty cheaply to store them. The one I have is even adjustable in length to accomodate different sizes of charts. The cylinder is easy to stow.
When making chart corrections, or annotations, use mechanical pencil only. When erasing, soft gum erasers only, so that you don't erase original print from the chart. If you navigate with charts a lot, use a mylar overlay taped over the chart. You can write and erase on that, all day long and not damage the chart.
Sorry, but this topic brings out the Quartermaster in me. The damn things are expensive enough, may as well make them last and get your nickel's worth.