Tips on how men's jackets should fit

E has to buy a suit for our wedding soon, and wants to make the initial selections himself and then have me help narrow it down.
Other than just vague ideas of "that looks good" or "no you can't wear 5 plaids to our wedding," it is nice to have some real information on what to look for and what to pass on.

This post advises you to be on the alert for shoulder divots, straining sleeves, creases when buttoned, collar gaps, and shoulder rumpling.