If you are having Invisalign treatment, you can download a great app that enables you to record your progress and to review the changes between each aligner.  Although I have not had Invisalign, I have downloaded this app and have tried it out and found it to be really easy to use.  I particularly liked the guidelines that appear on the camera screen to enable you to make each photo similar so that when you review the images, they produce a nice consistent time-lapse of your progress.