I bet we all have habits. From the littlest thing of biting your nails to the biggest thing of having no manners, we have them some where along the line. As you get older, people will expect you to have better manners. You shouldn't live up to people's expectations, but I do agree, having good manners are good.
Remember, there is a such thing as good habits. Like helping people whenever they need help.
Here's ways to break them:
Have a piece of paper in your pocket that says, "DON'T (fill in your habit here)!" You can write it in red, if you want.
Try to remember. If you have good memory, it'll be easy for you to remember not to do it.
Pretend there's someone interviewing you. They follow you around all over the place, and their streaming you on Live TV. You wouldn't want to bite your nails on live TV! This tip is good for people who have great imaginations.
Tell family and friends your trying to break your habits. Tell them that if they ever see you doing it, tell you to stop.
Give yourself a punishment every time you do your habit. Don't make it major, but something like, "No chocolate chip cookies for three days."