Verify that the plugs and equipment are compatible. If you are using slides, you should be able to use them with the lights on. Test this beforehand as you do not want to turn off the lights unless you absolutely must. If you are using video, again, verify that all of the equipment is compatible. The video should be no longer than five minutes.
Overhead projectors take practice to use effectively.
You cannot spend time aligning each transparency. Try not to use chalkboards. They are messy and difficult to read from a distance. You also do not want to turn your back to the audience, which is unavoidable when you use chalkboards. If you are using a flip chart, again practice many times before the event to memorize the order of the sheets.