I have a weak chin when my expression is neutral, but the chin looks perfect when I'm smiling. Is an implant a good idea?

Photos would really be necessary to give you the best advice. In general, a chin implant will:1) Improve the appearance of the profile2) Bring your facial features into balanceI would recommend a consult with a board certified facial plastic surgeon. He or she will evaluate you in person and advise on what will give you the best result. I hope this helps, and best of luck. 

Chin augmentation involves placing a Silastic implant through a sub mental approach directly over the bone, usually under local anesthesia. The implant  produces a static result, not dynamic. Computer imaging done smiling and not smiling may help you make your decision.

If you think your chin is recessive at rest you would probably benefit from an augmentation but Dr Eppley's recommendation of digital imaging  prior to surgery is a good idea to put your concerns to restDr Corbin