Here's a good method:
1. Select the teeth. I use the lasso tool but the pen tool or any other accurate selection method will do. Just be sure not to get any of the gums, tongue etc in the selection;
2. Feather the selection by 1-2 pixels to eliminate any hard line after recolouring;
3. Go to Image, Adjustments and click on Hue/Saturation;
4. Choose "Yellows" from the drop down Edit list and move the Saturation slider to the left to remove the yellowing;
5. Choose "Master" from the Edit list and drag the Lightness slider to the right until you get the level of whitening required;
6. Deselect and the job's done.
Big pearly whites: