How to write good luck messages
A good luck message is really a confidence boost. The best ones remind the person how ready they already are, like all the studying, training or preparation they've done, and tell them you'll be proud whatever happens. That takes the pressure off and lets them walk in feeling supported.
Send it the night before or the morning of, so it's fresh in their mind. For exams and interviews, a short, calm message works best. For big life changes, like a move or a new job, you can be a little more sentimental. Add a promise for afterward: a call, a celebration or a treat.