Shirt - make sure it is a clean shirt whatever the colour? White always looks the worst if old so wear these only if pristine. Check collar and cuff edges for grime and fraying. If possible stick to plain shirts or simple stripes so as not to let your outfit make statements about your person you weren't intending.
Wear a shirt without a pocket, or make sure it is hidden under jacket as best as possible, and also with double fold cuffs rather than buttons. Having cufflinks shows a man who takes care of his appearance.
Shirt style should be straight forward with a collar it fits. Much better to have a slightly more fitted shirt around the waist then a loose baggy mess around the neck. Medium spread as over narrow and huge cuttaways look out of place generally (again, this is unless they aid the frame of you face)