Photoshop tutorials:
Whitening Teeth
This tutorial is written for Photoshop, but it should be applicable to any image editing program.
Step 1:
Make sure you've done all your global enhancing first. Small edits like teeth and eyes should be among the last steps in your workflow.
Using Levels, I've lightened the whole photo to achieve satisfactory exposure.
Step 2:
Loosely select the teeth. I've used Quick Mask, but the Lasso tool would be just as good. The beauty of this technique is that no precision selections are required.
Step 3:
Make a new Hue/Saturation adjustment layer, and choose Yellows from the drop-down menu.
Step 4:
Move the Lightness slider to the right until you're happy with the result. Toggle the Preview on and off to judge your adjustment.
Remember, subtlety is usually best - if you go too far your subject will start to look like a cartoon character. Sometimes as little as 10-20% will be perfect.
Note:
In extreme cases where yellowness persists even after the Lightness adjustment, it may be necessary to desaturate the Yellows slightly, by moving the Saturation slider to the left.
I wouldn't recommend going further than 5-10%, because desaturation moves colours towards grey, and grey teeth are worse than yellow teeth!