Most practitioners get better results than their photographs suggest, because the photographs vary in everything except the client.
Fix the variables
Same position, same distance, same lighting, same background, same camera. Mark the floor. Use a fixed light rather than a window, because daylight is different at 9am and 4pm and different again in December.
Expression
Neutral expression, every time. A client smiling in the after photograph and not in the before is the oldest tell in aesthetics, and people recognise it.
Timing
Before, immediately after, at two weeks, and at twelve. The twelve-week image is the one that sells the treatment, and it is the one most often never taken.
Consent
Written, specific about where images may be used, and separate from treatment consent. A client consenting to treatment has not consented to Instagram.