Every debonair gentleman should know this.
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. Leaving all the folds loose will also help the shirt’s appearance, but only marginally as the state of being packed will inevitably press it to a certain extent.