RP Fic: Fox and Hounds
The main problem with Mary Sues is that they take up the entire spotlight in a fic. Canonical characters are pushed to the side and forced to act oddly to maintain the Sue's prominence in the plot. Her various angstful moments and sue-powers are also used to push her way to the fore of the fic and keep her there. Good discussion on her at http://www.livejournal.com/community/deleterius/338684.html, and also an essay at http://www.merrycoz.org/papers/MARYSUE.HTM
It's easy to start writing Mary Sues, but to go a step beyond them and develop good characterization is tough. Reading helps, certainly. However, a new story may well inspire as many new Sues as they stop. Plotbunnies for a Discworld/Lotr crossover might have to do with a meeting between Vetinari & Denethor, or the daughter of Aragorn & Arwen who needs to be trained by Granny Weatherwax. (Even the latter could be salvagable, if done right.) Reading also helps with grammar and logic, but that's another point altogether.
In addition to reading, I've found another method that helps somewhat is to get Sue-authors together in RP. If the Sue can work with an experienced, legible author, all the better. Just any old type of RP is a bit dogy, as it may turn into a bout of oneupmanship, but even then, there's the opportunity to practice character cooperation.
However, there was an old variation on checkers in which one player gets a single piece that must try to take all the others, the "fox," while the other side, with all its pieces, called "hounds" must try to trap the fox so that it can't move. This game was mainly to improve the second player's skills. Try picturing an RP similar to this: one author controls the workings of the Mary Sue and the basest generalities of the plot, and the other controls all the other characters. Not that I recommend having all the lesser characters team up to destroy the Sue, or that she uses her control over plot to get rid of everyone else, but by learning to build worlds for a character to interact with, the new "hound" author learns how to create characters who aren't the focus of attention, and becomes closer to learning how to write her own original world. If you're keeping a lot of characters in the small corner you have available of showing time, you can't really make a Sue. All of them are affecting the plot, and you have to be flexible. The "fox" brushes up on her story-weaving skills, at the same time. If the two writers are willing to work with one another, this can be a very useful role-play.