It's always good to be polite, no matter what the situation and this accounts for roleplay too. Nobody wants to play with a power hog or somebody who god modes. These are some basics, but these things vary from roleplayer to roleplayer. Best to play it safe.

Match the length of the post that is provided by the person that starts. There's nothing more frustrating to me personally (except for wretched grammar and spelling) than having my one to two paragraph posts answered by a five-word sentence.

That being said, please, oh, please use proper grammar. It's not too hard to hold down the space key and it makes your posts so much easier to read, especially if somebody has glasses or whatnot. Not to mention, it just makes your posts prettier. And grammatically correct to boot.

Don't constantly bother people asking them to post. Once, maybe. But if you keep asking somebody when they're going to post, it's irritating. I once roleplayed with a person that would, nearly as soon as they finished writing their post, start attacking me to post. I was writing. Some people type, or think up posts, faster than others.

ALWAYS follow the 'attack, not assume' system of writing fights. In this, you avoid power-playing by saying that your character attempted to attack another, but did not necessarily hit them, allowing the other person to determine what happens to their character. Example A: June lunged at the other man, slashing at his shoulder with her dagger. That was good. To say that she hit him would not be good.

If you're new to roleplaying, you may not know all of the terms involved. I sure know that I didn't! These are some of the most commonly used ones.

O.O.C./Out of Character: Out of character is something that the person writing the post, not the character, is saying.

Power Playing: Exercising control outside of your respective amount, like controlling other peoples' characters. This includes having your characters know something that is OOC knowledge.

God Moding: Making your character 'immortal' or something to a similar effect.

Auto hitting: Writing that your character automatically hits another and causes damage.

I shall be adding more, as I come up with more ideas.