One pin can change a plain jacket. Where you put it changes everything.
A Roman fastener
The fibula held Roman garments closed on the same spring principle as a safety pin.
Higher and off center
The upper lapel, the chest pocket, the shoulder, the waist. Never dead center on a soft blouse.
Three is the start
Group small pins tightly, in odd numbers, with one common thread: the same metal or theme.
Pin to structure
Jackets, knits and cotton dresses hold a brooch flat. Silk and thin jersey pucker.
From Maria's edit
Jackets that come already pinned. Start there.