Most men wear their shirt necks too tight, sleeves too short and wrists too wide which can make even a high quality shirt look all wrong. The collar rule of thumb is: with the top button closed, two fingers should be able to slide comfortably between the neck and collar of the new shirt. Remember, things shrink when you wash them so this adds just the right amount of extra room.
Have a seat in the dressing room when you try on your shirt to make sure it doesn't burst open like Superman. Maybe even dance around for good measure. You want your clothes to look great no matter where you are or what moves you're busting.
When wear a necktie, try it on and swivel your head around. Make sure the collar's points stay in contact with the shirts body. Semi-spread and cutaway collars should have no space above the ties knot, with points long enough to be covered by the jackets neckline. Lastly, no part of the collars neckband should show over the tie's knot.
The cuff should move with your wrist. You shouldn't be able to slide it on and off while buttoned, otherwise the circumference is too big.