I think I might add on to this. I couldn't really agree fully with all the points posted, but for the most part this is a good guide thread and everyone should look it through.
While this IS about roleplay, which involves you taking the role of someone you're not, I still think I should stress this point: Be someone you know you can be.
Don't play a character you can't pull off yourself. Don't play the sword-god if you're not tactful, skilled with PvP and have a good measure of common sense. Don't play the mage if you're incapable of improvising spells, understanding the concept of "magicka" or "mana" and utilizing it in a fair, resourceful and useful way.
And please, as said before, don't play "http://en.wikipedia.org/wiki/Shinobi" if you don't know what a "ninja" is. (The term "ninja" is actually untraditional.)
Don't play a http://en.wikipedia.org/wiki/Samurai unless you know a good lot about samurai and can write a decent conversion to an Akaviri equivalent. And please, allow your characters some weaknesses, or a lack of a certain profession.
My character, Akronos, will fight
like a samurai, but that's because I do know a lot about samurai, and I practice http://en.wikipedia.org/wiki/Kenjutsu and http://en.wikipedia.org/wiki/Jujutsu in real life. Does this mean you have to have some real martial expertise in real life? Not at all - it's just RP. But you should still understand the concepts of what you're trying to RP.
My character Akronos cannot for the life of him match a big wizard in magicka, and in fact carries a pendant that increases his potency in casting, but a sword that nullifies those energies when drawn simply to compensate if faced against casters. I am not the king at RP spellcasting, so I wont attempt to do it.
As you can see, I play MY strengths, which helps the RP as my character is what you expect him to be. However, I try to keep him balanced, I don't have him do things he/I don't know how to do. Try to keep things simple if you can, too.
Having one thing you're really good at is kind of neat. Two is alright. Three, I guess. Four, you're pushing it.
And remember, unless you're REALLY good,
jack-of-all-trades just means you svck equally at everything. Oh, and please, spellcheck. It's there for everyone - it doesn't take long to fix your mistakes. Some really good RPers I know just aren't much good at spelling, or maybe English isn't the native language. Just take that extra step to make sure your post is legible.