If you are a keen walker, you might find it worthwhile to buy a transparent plastic map protector- it will keep your map dry on even the wettest outings. Sturdy examples with a zip top are available from most camping and walking equipment outlets.
Or you can make your own with two sheets of heavy duty transparent plastic, each the same size, secured around three sides with insulating tape. Leave one side open so you can insert whichever map you need.
If you regularly visit the same area, cover the relevant ordnance Survey map with a piece of adhesive transparent plastic film, as if it were a children's exercise book, to keep it clean and dry.
