Persistent followers of my blog know that I often get my drawing practice while visiting my local coffee shop. I am often asked if I have frequent encounters with angry drawing subjects demanding to know why I'm staring at them, but the truth is I've never had anyone get upset. I've only been met with curiosity and enthusiasm. The secret is... don't act like a creep! This sounds simple, and in practice it is, but it takes some thought to understand it from your subjects perspective.
Number one, don't hide and don't hide your drawing. It may seem counter intuitive when you don't want your subject to be aware that they are being drawn (since this can make them assume a less natural and more self-conscious pose) but acting secretive equals acting creepy. Be open, be friendly. You don't have to tell your subject that your drawing them, but if they happen to come over or walk by, don't cover up your drawing . Show it to them if you like or just make it easy to see so they know what you're doing.
Number two, don't be distracting. This is the most common mistake artists make when drawing in public. They put their sketchbook flat on the cafe table and continuously bob their head up and down between subject and sketch. Movement in our periphery vision catches our attention, especially when it's followed by a face looking at us. So just as the left side of the animated image in your periphery vision (below) is distracting you from reading this, the artist distracts their subject with constant movement and draws the subject's attention to the fact that they are being stared at. Solution: find some way to situate your drawing surface and the subject in or near the same field of vision so that the only thing you have to move is your eyes. This is different from being secretive. It's a matter of common courtesy to avoid being distracting.
Don't be distracting to your subject!
Even using this method, I am sometimes "caught." I glace back up from my sketch to have my gaze met head on by the subject before the drawing is finished. Not wanting to disturb them from their pose, I change the direction of my glances to something else nearby for three or four glances. This is usually enough to make them think I'm actually drawing something else. When they go back to what they were doing I continue my drawing. Then when I'm done, I leave it out on the table for them to see if they decide to come investigate with a walk-by; sometimes we have a conversation.