This is pretty simple. If there is more than two of you in a thread, try and keep the post order. Basically this gives everyone the same chance to rp, and also will give yourself more to rp with, as you have all the people behind you to respond to.
So say I (A) was posting with two other people (B)(C). Simply put the post order would be (A)(B)(C).
There are times when this may not be able to happen because one of the other people 'go away'. In this case, you should post between the people left behind, still in the same order. Unless given permission, then don't do anything with the person that is away. Moving/Making feelings known/Even hurting is seen as godmodding which is frowned upon.
Patience is a virtue in the rping game, sure you may wanna post continually, but sometimes you will just be posting for the sake of it, and the quality will probably be down
Keeping a post order also keeps you from 'taking over' the thread with someone else. If you are in a thread with say five other people and only you and someone else are posting back and forth and not allowing the others to rp, it can be seen as 'taking over' and making the thread about yourselves, this can be seen as mary/gary sueish trait, again, this is frowned upon, even more so than godmodding.