Moving from one karate belt to the next is quite an accomplishment. Since you won't be using each belt once you graduate out of it, your options are to either tuck the old belts in a closet, pass them on to someone else or display them with pride.
A Folded Display
Fold the belt in half three times; the resulting bundle should be about as long as your forearm. Pound two columns of metal pegs into a backing board of your choice. Karate belts come in various sizes, so measure your folded belts and separate the pegs just enough to slip a folded belt between two neighboring rows, with the face of the belt facing out. If you're feeling fancy, use a scroll saw to cut designs into the backing board.
A Rolled Display
Prep your belts for display by rolling each one into a tight spiral, starting from one end. Stack the belts one on top of the other inside a cylindrical glass jar -- often, a glass vase is your best bet for finding just the right shape.
Another Simple Option
Girth-hitch your belts to a wall-mounted dowel; a short curtain rod; or a short, elegant-looking towel rod. Fold each belt in half and hold it by the middle; position the middle of the belt just below the dowel or rod. Pass the two ends of the belt up between the rod and the wall, then down through the loop formed at the fold in the belt. Tug the ends of the belt to snug the loop up against the rod.