Before I tell you about my third maxim (which is: act quickly as needed), let’s talk about affect.  Strict neutrality of the face ends up looking hangdog and depressed, which alerts other people to your presence. That means, strategically speaking, to fly under a person’s radar, you’ve got to make a wee bit of eye contact and you’ve got to crinkle your eyes a bit.  That makes you look just pleasant enough to be unremarkable.  Also, keeping perfectly still is a bit of a tip off.  It’s better to have just the slightest body tic; twirling of the hair, for example, or a very small tap of the toe. Just a little something to stay invisible.