Your best bet for crisp, crease-free shirts is to iron them while still slightly damp to more easily reduce wrinkles. Always use the appropriate heat setting for the fabric, and iron on the reverse side to prevent shine marks. For delicate fabrics like lyocell and silk, keep it low temp. More structured fabrics like linen can use moderate heat, while cotton, denim, and wool can withstand higher heat. You can find more details on specific fabrics on the tag.