My chin is weak when I smile, but seems okay when my face is relaxed. What can I do?

ideally the angle between the jaw line and neck should be a sharp 90 degree angle.  The impression of a weak chin can be from excess fullness under the chin.  In a female the lower lip and chin should be in alignment in the profile.  It could be that you might benefit from subtle jaw enhancement with fillers and fat removal from under the jaw line but it is always hard to tell in pictures.  I recommend you see a board certified plastic surgeon and make a plan that may help achieve your goals.  

Dear Marie,Thanks for the pictures. It appears that the lower third of your face is a bit deficient. That  deficiency revolves around the lower jaw and chin.  The best way to improve  this would be to have lower jaw and/or chin surgery.  Best wishes 

 The limited photographs demonstrate a recessive chin profile statically  At rest  and dynamically when smiling.  Consider placement of a small chin implant augment the chin forward.  Fatty deposits in the neck can be removed with liposuction which is also complementary to the jawline and chin implant. A chin implant and neck liposuction can be performed under local anesthesia as an outpatient procedure.