The key to a good demonstrative aid is making it memorable for the jury--especially since the jury will be unlikely to take the exhibit into the deliberation room at the end of trial.
Try to involve as many senses as possible: seeing, hearing, even touch.
Example: In a case involving a plaintiff crushed by a beam, have the expert drop the beam so that jurors will get an understanding of the beam's size, weight, and danger.
For written exhibits, use pictures as well as words. Pictures are easier to remember than words.
Pictures alone without written words--supplemented only by the spoken words of direct testimony--are even better.
Source: "Add Dynamics to Direct Examinations," by Thomas J. Vesper (Trial 9/03)