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.
Don't be distracting to your subject!