But there are pitfalls to avoid. First of all, the most common pitfall, is the habit of turning your back to the audience and talking as you write on the blackboard. The negative effect is twofold. Not only does the speaker not project his or her voice as well, doubly problematic when the speaker's English speaking skills may be less than perfect, but also he or she loses contact with the audience, which no longer feels the speaker is engaged with them, but with the project of writing on the blackboard. You should also be able to write very quickly, legibly and large enough to be seen from the back of the room.
Organization is another important consideration.
Start on the left side of the board, and work to the right. Don't erase until you have asked the audience if they are finished copying from it. In order to avoid spending valuable time writing on the blackboard, you can use overhead transparencies which you can prepare ahead of time. There are some rules for their use too. First of all, when designing your overheads, try not to include too much information on one transparency.