Be patient
When you post something, it's the other players turn. Don't continually write twelve lines while the character is in the progress of responding to what you wrote before. Good roleplay is basically taking turns at any length. If you continue to spam one-liners it doesn't give the other player a chance to respond.
Self-insertion
Sure, it's fine to put a piece of yourself into the character you're playing. I have known some good role-players and excellent writers who have basically been a self-insert, but... If you're playing yourself in an unfamiliar situation, you're not really playing a character. The more you separate yourself from your character, the easier it is to keep the lines between yourself cleanly and neatly divided.
Overattachment to character
I think many are guilty of this sometimes, especially when you are relatively new to roleplay. You are writing with other people; they are not opposing you and they are not there to make your life miserable; they are (hopefully) trying to act how their character would act and immerse themselves in a different world.
People do not do what you want them to, scenes will not always go the way you want them to. When this happens, when your own feelings about whatever happens become blurred with the characters, return to character. Some of the best scenes can come from spontaneity and randomness. If something happens to your character that pisses you off, take a deep breath and think of what your character would do next. If breathing isn't deep enough, maybe it's time for a break as soon as the scene finishes.
Perfect Characters
No-one in real life is perfect, and one should strive to some level of realism in their roleplay. Your character feels pain, your character gets angry, has emotions. Has flaws, has a backstory. Don't make the character that everyone is supposed to love without giving other people a reason to want to roleplay with you.
Metagaming
When you get surrounded by bandits, as tempting as it, don't get on steam and summon your sixteen friends immediately. If you go off alone in a military situation or your character pissed someone dangerous off, don't summon your sixteen friends. It's unfair to the people who are taking the risk of confronting your character and creating an interesting tense scene for you to enjoy. If someone robs your character, go find someone IC and hunt them down. If someone hurts your character, report them to authorities or plot your revenge depending on your character. Just don't meta-game.
IC seeping into OOC
Tense dramatic situations between two characters are fun if approached right. If you get wrapped up in whatever scene your character enters, just say nothing OOC at all. If rules get broke and finish the scene and report them afterward. Maybe you can state that they are breaking the rules and why, but in a tense scene this just ends up delaying it and makes it frustrating for yourself and the other party. If it's obvious that talking won't help or that they are not willing to discuss it, finish the scene and contact an admin or write a complaint. In hostile situations, it's sometimes best to start recording or taking screenshots just in case.
OOC seeping into IC
You will not get along with everyone, you will not like everyone out of character. You should try your best to forget the name behind the character and act according to your character towards theirs, If you suspect a character is behaving a certain way towards you because of OOC, avoid them. If they continue to assert themselves on you consistently, report them.
Over-passivity
It is no-one's responsibility but your own to create interactions and stories and plots for your characters. If you're having an issue with this, it's an issue with your creativity, so you should search for inspiration.
Overly aggressive
As tempting as it can be to throw your character on every passing person without an IC reason, don't do it. People have goals, have other things their character had planned, have ongoing scenes. Don't force interactions where it wouldn't make sense because of your boredom. In a similar vein, if there seems to be a one-on-one RP going or a small comfy group RP, you don't always have to intervene. Depending on the situation you might be welcome, but if you're not sure, don't. If you are new, sometimes you have to be assertive but still, be mindful of what other people had planned.
Playing to win
As many Warband players are competitive, it can be hard to allow your character to lose or to want bad things to happen to your character. Allow your character his failures and allow for their redemption arcs which might be more interesting than if everything had went your way. Sometimes, the best RP comes from failures. Similarly, throw some accidents into your RP to create more dynamic situations.