I probably don't even have to mention this, as it's most likely a no-brainer for everyone here on the forum, but for many of the games I play (via my USB stick), I try and reserve one of the C64Mini's save states for recording my high-scores.
There are not many classic C64 games that save high-scores to disk. For those games, if I want to preserve the leaderboard, I ensure that I save the game-state after I've registered my score in the hi-score table. This way, I can restore the game the next time I play, keeping my scores intact. Again, this is probably something that everyone else does, but I just thought I'd mention it in case there are newcomers that may not think of using their C64Mini's save game feature in this way.
Also, don't forget to take a picture or screen-grab of some sort of your high-scores and submit them to Retro Computer Scene's High-Scores Archive!