"Natural" in photos is a skill, and like any skill it has shortcuts. These are the ones I use with clients every week.
Before the shoot
Sleep well, wear clothes you have worn before, and skip the brand-new haircut. Comfort photographs better than novelty.
During the shoot
Keep moving
Micro-movement is your friend: shift your weight, turn your shoulders, walk a few steps. Frozen equals awkward; motion equals alive.
Breathe out on the click
An exhale relaxes your jaw and shoulders — the two places tension shows most.
Think of someone, not something
Instead of "smile", think of a person who makes you laugh. The difference is visible in every frame.
After a few minutes
Nerves fade with evidence. Once you see the first good previews, the rest of the session takes care of itself.