Start by ironing sleeves, cuffs, belts and collars before ironing the body of the shirt. This will prevent you from having to re-iron the body of the shirt which is bound to get wrinkled when you iron the smaller part of the garment.
When ironing collars, iron from the corners inward to prevent wrinkles on the ends of the collars. Be sure to remove shoulder pads from shirts and jackets before trying to iron. If you leave them in it makes the shoulder areas very difficult to iron.
I've found the following helpful: a) Damp shirts ironed without steam obtain a crisper finish. b) Sizing vs. spray starch gives a 'new shirt' finish. c) 'Belts' &/or 'shoulder pads' in a man's shirt is very suspicious.