Always accept as fact what the other person says. Then add a bit of info to it. So if someone tells you that he's ready to fight against an evil goblin army, then acknowledge that info and build on it. Keep doing so and you'll end up creating a fascinating storyline that continuously grows and expands.
Try to use short, one-line messages. The shorter the better. Entering messages in quick bursts lets the other person respond in 'real-time.' Can you imagine waiting a long time while your peer was typing up a huge message? What could you do, other than sit and wait? How could you possibly add to the story if you have no idea what huge message your peer is typing up? For these reasons, feel free to rapidly fire out short messages
Don't start with a character called 'sad sheriff' or 'boring teacher'. Those kinds of lackluster starts usually lead to boring scenes. Instead, pick character names such as 'Sam the Sheriff' or use adjectives such as 'tiny sherriff' or 'teacher from Mars.' The same goes regarding your first statements. Don't start a scene by saying 'you are weird and uncool.' Instead start with something exciting like, 'I LOVE your clothing style!