On a two, three or double breasted button jacket, always leave the bottom button undone. Fastening this will mess with the drape and fit of the jacket. Leave this undone, always.
Always unbutton your suit before sitting down. For comfort and it adds unnecessary strain on the button(s). Give your suit and yourself some love.
Contrast your pocket square and tie with your suit. Don’t wear a blue pocket square with a similarly shaded suit.
Socks should always cover your legs. Keep in mind to not show any leg skin while you sit down, your socks should always be long enough cover your legs.
Don’t over accessorise. Have enough accessories for the occasion. Wearing a tie, lapel pin, tie bar, pocket square, bracelets, is a little overboard. Keep the accessories to a minimum.