Most of my IC/OOC confusion comes from mixing up thoughts and dialogue in whatever post to which I'm replying. I think it's important that people only really respond to words said and any explicit actions/expressions, so it helps if whoever I'm playing with makes it obvious (like bolding dialogue).

I also don't hesitate to ask or have another RPer remind me of what my character would know about a certain situation. Talking really comes in handy, especially when you're not RPing an interaction and just assuming that it happened. Communication between RPers is most vital, and it clears up a lot!