Pat Winston in his lecture on How to Speak notes that covering up parts of overhead transparencies and revealing them slowly like a strip-tease artist is a technique that drives 10 per cent of your audience nuts. I am in that 10 per cent. The desire to use this technique means only one thing: There is too much information on the slide. Split it into multiple slides. Winston recommends using overlays instead, but overlays are really a different and specialized overhead technique, and are not typically necessary for remedying this problem.
By the way, if you make slides using computerized means and want to use an overlay, consider “implicit” overlays instead. An implicit overlay is a series of separate slides each of which includes the contents of a different prefix of the overlay slides. Implicit overlays have the advantage that no Scotch taping of slide material is required, and no fumbling with the overlay pieces is needed. One just continues placing single sheets on the projector as usual, but each one in the overlay series has some additional material added to the previous one.