Be sensible about transparencies
The overhead projector has in the past decade become the visual
aid of choice for physicists. Experienced speakers often have
strong opinions about the optimal number of transparencies for a
talk—typically 3 to 6 for a 10-minute presentation, 10 to 15 for a
half-hour talk. Although you needn’t adhere to hard and fast
rules, as a practical matter you should allow several minutes for
an audience to absorb each transparency. You’ll thus want to
avoid transparencies that have only one or two lines of
information, just as you’ll want to avoid those so jam-packed
that they overwhelm your audience.