Gangsters and the mafia have over time become icons of American culture that have surpassed their actual origins. During their heyday, they dressed like anyone else, but images in movies such as "The Godfather" and on television have caused that look to be immediately associated with gangsters and the mafia. The difference between a gangster costume and a random guy in a suit is a matter of getting the details right.
Things You'll Need
Hair gel (for people with long hair)
Black collar shirt
Black dress shoes
Violin case (optional)
Longer hair should be slicked back with hair gel. Try to use a product that gives it a greasy shine.
Tuck the black collar shirt into the suit pants. Secure the pants with suspenders. The shirt should be bottoned all the way to the top.
Tie the tie. After finishing, poof out the tie below the knot as much as possible.
Put on the suit jacket and button it up all the way.
Fold the handkerchief into a triangle and put it into the jacket pocket. It should not be ruffled, but should lay flat against the chest.
Put on the fedora and shoes. Optionally, carry around a violin case as well.
The closest thing to informal for this time period would be removing the suit jacket. The hat should be worn almost all the time, as well. This is a period with a very proper dress code.