Don’t respond to every bit of speech.
Don’t be afraid to give non-verbal responses, nods, stares, shakes of the heads, funny looks, waves of the hand, thumbs up, smiles, grins, shrugs, crossing of the arms, and so forth. This simplifies the thread and can help prevent awkward speech patterns between the roleplaying characters.
Don’t immerse yourself completely in the character’s head. It might be great that your character is thinking of her dead parents in this somber moment, but it gives the other roleplayer very little to reply to. Make sure your post doesn’t consist solely of thought, it’s very difficult to reply to.
Don’t overdo the action, either.
Don’t over-stuff with action, changes, and alterations. A slight change of scenery, like the sun beginning to set, is great. A major shift, such as a cliffside cave beginning to flood, may not be so appreciated by the other roleplayer(s).
Don’t be over-controlling.
It’s important not to entirely direct the course and flow of a thread. Allow the other player to make some decisions, even if it’s an unplotted thread, this is easily done by leaving open-ended replies. For example, if two wolves are hunting a moose, the first character’s reply could detail their approach, the second could detail the selection of suitable prey, the third could detail the actual attack, so on and so forth. Each roleplayer gets to dictate a different part of the interaction and advance the storyline a little; it’s more fun for everyone this way.