In addition to joining contests, playing the games will eventually get you there as well. It takes a lot of patience. See THIS for approximate point breakdown between levels, especially if you're shooting for the secret characters.
Like everyone else has been saying, it's rather easy to get to 6th Super Silver. After that, it can get pretty agonizing. Playing a mix of games and doing your best in the contests works best for me. If you get good at them, you'll get more CD Experience, and level up faster. It can take a good deal of time- in fact, it will- but you'll get to the rank you want eventually!