Like a lot of people who are usually hiding behind the camera, I find it hard to be comfortable in front of the lens. You might be able to master 5 point lighting techniques and somehow understand the zone system for perfect exposure, but trying to smile naturally for the camera is one of the most difficult tasks I've come across. The old "say cheese" trick doesn't make you look very happy...maybe a little scary if anything. Thinking happy thoughts...well, that can only get you so far.
Luckily here's an informative video that can potentially help remedy the duckface you've resorted to. From personal experience, I've found that giving a little chuckle helps...even though it does make you look a little crazy in the process. It loosens up the stiff muscles in your face and gives the the general impression of someone being happy instead of excruciating pain. Other tips like saying things that end in "uh" can help produce a natural smile...getting doped up before a photo shoot is not productive however.
