The most common issue that I see with regard to smile shots is the fact that they are either off center or aren't angled correctly in a vertical dimension. By this I mean that they look as if they are either being shot from above or below.
Here is an example of an image that was taken from below. It's the most common mistake, and far more common than the error of being taken from above. Why is that the case?
Most dentists and assistants make the mistake of taking the image of the smile with the patient reclined far too much in the dental chair. As a result, just from a postural position, one must either get the patient to lower their chin a great amount, or have the dentist stand on some device (not recommended due to danger of falling) to capture the proper 90 degree shot. Lowering the chair is usually not an option as most individuals aren't tall enough to get a great angle even when the chair is at its lowest position.
If the chair is upright so that the patient is nearly upright, then there is minimal adjustment necessary. It's also important to observe your body position while shooting the image. You should be standing on the side of the patient at around the anterior/posterior level of their wrist and have the patient turn towards you. A lot of dentists lean out over the patient and that's a sure way to a tweaked back.
Last, but not least, look through your viewfinder and compose the image perfectly while asking the patient to smile. Once you've properly framed it, ask the patient to relax, all the while trying to keep you focus on the smile that will appear. Count down from 3 and then ask the patient to smile. Capturing the image immediately will ensure a natural smile shot.
Here is the same image from above, except taken at the proper angle.
Good luck, and please contact me if you have any questions or topics that you would like to see discussed.