Automated simplification of trigonometric expressions is an important problem that hasn't been completely solved by current computer algebra systems. This paper presents a number of unique prescriptions for the ordering of some trigonometric transformation rules, which have been derived by observing how human experts follow their intuitive rules. We have implemented the procedure in Lisp because of its suitability for formula manipulations and rule-based reasoning systems. Consequently, it can simplify many trigonometric expressions which are even diiffcult to do by hand, and it achieves much better results for many hard problems than any of Maple, Mathematica, and Maxima do.