Face the Class

Don't speak with your back to the class!

It's simple. If you are writing on the board and you have your back to the class they will not be able to hear you properly. Don't speak when you have your back to the class; only speak when you can face them all and they can hear you well. This will also allow your students to see your body language and any gestures you need to make to them.

Oh, and also remember to clean the board when you've finished for obvious reasons!