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Why don't you do it like Miniclip? Basically there are high score lists for the current day, the current week, and the current month, and that means it's easier to get higher on the daily high score list, slightly harder on the week, and to be the best of the month... "
But that means that if you get an amazing score, it's not held up there forever - if you want to stay on the highscore board, then you have to play the game and get that score at least once a month. It's better to have an 'All Time' section - that way, you can be a legend forever (unless obviously someone beats your score).