This thread made me laugh!
As a small, smiley, gamine kind of person I'm used to being read as younger (when I was between the ages of 15-50 -- now, the problem is how not to come across as "little old lady, ha!); "cute" (at any age), and nonthreatening.
But I also taught secondary school for a while. Shevia's method worked best for me. The stare, glare, raised eyebrow, and shifting voice register have proved useful.
For clothing -- nothing frilly, cutesy, overly revealing. Usually a jacket with sharp shoulders and boots.