The trick to in-game RPing is to remember that ninety percent of what you'd normally have to describe or say is already drawn on screen for you by a graphics engine. You reduce dialogue to 1-2 lines in most instances and while it's fine to have the occasional action (ex: /shrug, /laugh, /chuckle, /pat, /me has a confused expression, etc) it's best to not spam this either, it makes you sincerely look like a tryhard.

Also, that a lot of people you will meet in MMORPG role playing communities are just plain, truly atrocious at it.

Beyond that? If you can find a solid group, remember it's all about the speed. Pacing is a friend to any story and if 90% of everything you'd describe is done for you, then MMORPG's are much less about forethought and description, and far more about rapid thinking and quick but meaningful dialogue.

That's about all I could offer as a suggestion, really.