How to make eye-contact and smile the right way, not look like you're staring someone down? Guys and girls?

So yeah, I'm a little bit afraid that I'm too overbearing and that my eye-contact could be a bit more than intended. I don't want to seem like I'm staring at him or anything. So, what's the proper way to catch someone's eye and smile, or how do you do it to catch someone attention?