Some professors like to simply use line drawings directly from a book or journal.
Traits of journal illustrations that produce disappointing results are fonts that are too small and lines that are too fine. Make your overhead on your local copy machine, try it in the room in which you teach and look at it from the back row and sides. Many disciplines have sources of clip art and fonts that are pertinent to a subject, such as a periodic table for chemists or mathematics symbols for engineers. Clip-art outline maps are also worth owning. Pie-chart overhead produced from journal data.