I have found that wearing a suit and tie is very effective if done occasionally with non-math-oriented students. It tells them that I care because it shows that I have taken sufficient time to prepare and teach the class even though I am a busy person whose schedule requires meetings with:
With math-oriented students, however, a tie tells them that I spend too little time thinking about isomorphisms
Cf. Focus. Professors were once students.
some powerful political figure,
some powerful economic figure,
some powerful university administrative figure, or
some TV interviewer