Don't mind me, I'm just scratching my head.

You know what is the most embarrassing thing? When you're out in public and you see someone you know. You think they see you too, so you wave. They didn't see you, they don't wave back, you look like a loser, standing there waving.
What do you do in that situation? Keep waving in hopes that they'll eventually see and wave back, so that you can prove to everyone that you're NOT in fact a loser? Usually I'll pretend that I was just stretching or scratching my head or something, both of which don't really work that well, but fuck. I'll be damned standing there waving at no one.
The other side of this situation is waving at someone you think you know...but then it turns out that they weren't the person you thought they were. This poses a different problem...I have two solutions. When they are looking at me like "Who the fuck is that?" I either go on my tip-toes and pretend to wave over them or past them. Then when they turn to see who I'm really waving at, I run away. The other option is to just follow through...wave at them and make them think that they SHOULD know who you are, and then for the rest of the day they'll be racking their brains wondering who the hell was waving at them.

This video clip, from a show called "I'm Alan Partridge" (amazing hilarious show by the way) demonstrates what NOT to do.
Enjoy.