どうぞよろしくお願いします　is the most polite way. 「どうぞ」means "please" in this context. It just makes this phrase more polite. よろしく is the shortened form of よろしくお願いします、 and it is often used between friendly relationships. To your friends, or in a situation where you are asking your friend a small favor, you will often use よろしく。 To your seniors, or in a situation you are asking somebody to do you a big favor (even if s/he is younger than you), you would say よろしくお願いします。 I hope you understood a little better about this. See you! If you have more questions, please reply to me, よろしくね！