1. Get To Know Your Photographer
It can really help to have a photographer that you know and that you are comfortable with because you will naturally be more yourself while taking the pictures. Although, if you don't know your photographer, try to get to know them. You may find you have something in common with your photographer, ultimately helping you to not look stiff in pictures. The more authentic and open you are with your photographer, the more willing they will be to try new thing and get you out of your comfort zone, which a lot of times are the best and most creative shots as well.
2. Practice Poses In Front Of Your Mirror
It is a weird thing to do but if you can't model in front of yourself in the privacy of your own home, then you are going to really not going to do well when there's someone in front of you with a camera up to their face ready to take your picture. So give yourself some practice if your camera shy.
3. Do The "Fake Laugh"
I learned to accomplish that "fake laugh" by coughing. It works for me and it usually leads to me actually laughing because it is a weird trick.
4. Figure Out What Side Of Your Face/Body You Like Better
Most people aren't 100% symmetrical, so there is one side of you that you may like better or that you feel photographs better. It is helpful to learn yourself and figure out at what angle and tilt look best for you.
5. Research Poses
This may sound a little weird, but it can be helpful to create a board on Pinterest or on your camera roll of poses that you like of other models for inspiration. At times it can be hard to "be natural", so if you have some ideas at hand you will feel less nervous.
6. Be Patient
Sometimes the first shot ends up being the best one, but oftentimes it's not. Be patient with yourself and just let your photographer snap away.
7. Slight Movements Are Key
It really helps your photographer if you sightly move in between shots. This will help to give them more ideas as well because you really don't need hundreds of the exact same picture. You already took it once, if it is good then you got the shot but, otherwise, you need to change something to get a better angle, lighting, positioning, etc.
8. Use The Soft Smile
My trick to the soft smile is to press my tongue to the top of my mouth. For me, this helps me to not over smile and show my gums.
9. Keep Up A Positive Attitude
Have a good attitude and be willing to try new poses, even if you are uncomfortable. Also, play fun music! It can help lighten the mood.
All the best to you and your picture taking adventures!