On a lighter note, I have often found that the best way to be invisible in a useful or fun way is to work on the fact that if someone isn't looking at you they can't see you. If nobody can see you then by my (admittedly twisted) logic you are invisible.

This is a theory I try to use in everyday life. I have learned that whilst it is usually very hard to remain invisible to everyone, it can be very easy to remain invisible to one or two people. A perfect way to escape from that old "friend" you never really liked. Stay behind them, tiptoe, and try not to giggle. Ignore the fact that everyone else in the shop/street/whatever is giving you dodgy looks - they cannot be expected to understand. Best attempted in large groups for a real challenge, although it is possible that you may be arrested for intimidation if you get spotted.