Smile back.
It may seem obvious to you but this did not occur to me for the first few years that I was a portrait photographer. One of the best things that works in getting subjects to smile at me, is for me to smile at them. For years, I didn’t really smile when I stood in front of my subjects and then it occurred to me that if I just started smiling at my subjects, they would probably start smiling back.
Just try it. While photographing your subjects, keep smiling at them naturally. Look happy and smile at them as you have them say something funny or just while you talk to them. Also try complimenting them as you take their photos. This puts people at ease as well. The interesting thing about these two simple suggestions is that it works on people of any age, from babies all the way to senior citizens. It breaks down the nervous barriers people have about getting in front of the lens. Smiling is contagious and it will make your job a lot easier when it comes to getting pleasing expressions from your customers.
Practice on anyone, not just someone your pointing a camera at. Go now and just smile at someone. Chances are, they’ll smile back at you. If you know this person or they are a complete stranger to you; it will make no difference, they’ll smile back. Or just try telling someone how much you like their smile, all the while smiling at them. You’ll be shocked at how easy it is. Are you smiling yet?