When there is a board that you will write on, you should always stand on the left from the perspective of the audience to the board. When writing on the board you shouldn't have your back blocking the audience from seeing what you are writing. When showing a presentation, you should stand on the side from which the content language is read. English is read from left to right so you should stand on the left to make pointing to the content more natural. The center of the stage is an attention-grabbing place to stand and is usually a place for announcements. The left and right sides of the room, but not close to the walls, are considered power positions, you are able to see and communicate with the entire room without any head movements.