The basic but important thing that a well-dressed gentleman must know is to button his suit the right way. Not all buttons must go in the button holes, and here are the only four rules you need to know.
With One-button jacket: While standing you must button the only button. When sitting, you must unbutton.
With Two-button jacket: While standing: top button must remain buttoned, while bottom one is left undone. When sitting, you must unbutton.
With Three-button jacket: Follows the rules of SOMETIMES-ALWAYS-NEVER. Top button is optional (button if you feel like it) middle button must always be fastened, and bottom button should never be fastened. When sitting you must unbutton.
With Double breasted jacket: Keep everything buttoned except the bottom one. Unbuttoning is optional while seated, so long as the fit allows you to be comfortable.