Knock before you enter a room and be respectful. The little book of gentlemen's rules need not thrown away. And in the age of gender equality ladies can adopt it too. Pull a chair for the person next to you. Open the doors for people behind you and so on.
Chivalry never fails to work it's charm.
People love to be remembered and acknowledged. Remember people by their name. It is okay to identify the girl next to you in English class as Blondie for a few days but after that it is imperative that you know her name and use it. Look people in the eye when addressing them. Apart from making them feel you are confident it also prevents their attention from meandering.