These were found on another site, but I thought they might come in handy...
Sometimes a short post can make your character seem a little...flat. There's no substance, nothing to describe how or why they're behaving the way they are. Take the time to flesh your posts out a little. What is the character thinking, or feeling? Why are they behaving this way? Don't be afraid to be more descriptive, it gives the readers an idea of your character's personality. And when we say show emotion, we mean it. Let them get angry and throw a tantrum, or let them bounce off the walls with happiness, or drop to the floor and burst into tears. Breathe some new life into them.
Take the fall
As nice as it can be to have your character come out on top of a situation, it gets old for your partners when you try to best them every single time. Show vulnerability. Let your character lose a fight, an argument. Let them not know the answer to a question or have a witty comeback to every insult. Have them fall down the stairs or walk into the wall. Little things like that make them more human
Go with the flow
Sometimes things don't go according to plan when you're playing out an idea. Don't panic, don't get mad. Just go with it. Seeing the way your character reacts when put on the spot can be so much more interesting than if everything is scripted word for word. Sometimes you just have to wing it. Take advantage of the challenge at hand.
Find a balance
There are two types of Mary Sues:
Type A: The drama queen
Something is always wrong. Her parents hate her, her boyfriend broke her heart, nobody likes her, the world is coming to an end, and so on and so forth.
Type B: "Princess Sparklypoo"
Everything is perfect. Her parents are perfect, her boyfriend is perfect, everyone loves her, her grades are outstanding, her hair is extraordinarily shiny today...
We've all seen them, and we've all wanted to beat them to death with sharp implements. If you can find an even balance between these two types, you'll do fine. Keep in mind, Gary Stu is Mary Sue's twin brother.