The secret is to fold the shirt in half rather than in thirds. In other words, when the shirt is on its front laid out for folding, turn the sleeve sides over only a quarter of the way, leaving a space between them. Then fold the bottom up so the shirt is roughly halved into a square.

This way any creases that the shirt may acquire while packed will not be visible when you have your jacket on.

How to Fold a Dress Shirt