A Latin phrase I like
loquendum ut vulgus, sentiendum ut docti
Speak like an average person, but understand like a learned one.
Come to think of it, Rudolf Flesch said this even better, in plain English:
[I]t's a good rule to know as many rare words as possible for your reading, but to use as few of them as possible in your writing.
Rudolf Flesch, How to Write Better 35 (Science Res. Assocs., Inc. 1951).