So there are quite a few I could list. The main ones I'd say are:
Not being themselves is a common mistake. It's important to relax, be genuine, and enjoy the experience rather than worrying too much about posing or looking perfect in every shot. Turning up on the day with no thought or not making any effort and following the advice above generally leads to regret. Espeically around the issue of clothing, hair and makeup. Trying untested beauty treatments or fake tanning / sunbedding right before can leave your skin red and blotchy