You wait and watch and thump that watermelon every day, but you are still not sure if it is ripe.  Well, you wait for a windless day, and lay a broomstraw horizontally on that watermelon.  If the broomstraw begins to move back and forth, the melon is ripe for picking.  This really does work.  But, if the stem of the watermelon where it attaches to the vine, has begun to wither and die,the melon is ready to pick.  There really is no way to preserve watermelon, although it will keep in the refrigerator for about a week, and you can freeze small, seedless chunks for smoothies at a later date.  Or you can just sit on the stoop and spit warmelon seeds.