You can use a very hot iron to press your shirts – and this works best.  It is ideal if the shirts are a little damp, but not too wet. Do also use a shot of steam from the iron (again irons steam better when they are really hot).  It’s really important, after steaming, to iron without steam to ‘dry’ the fabric otherwise creases will appear from just moving the shirt around the ironing board.  On the same note, it really helps to put the shirt on a hanger after ironing to cool and dry before wearing.  Finally, if you don’t already, I would advise to use a good fabric conditioner in you wash which coats the fibres to help minimise creasing.