How can a man look stylish in casual clothes?

Casual outfits can be stylish as well.

Follow the rules below to impress:

Wear well-fitted clothes.
Wear some kind of accessory - a bracelet, a chain or a stylish watch.
Wear a jacket with details ( like my US Polo ).
Trim your beard, and keep it well-styled.
Your footwear needs to match your attire, and suit your personality.
Do not be afraid of color. Wear what suits your skin tone, lifestyle and professional decorum.
If you are transforming/upgrading your style, do it gradually. Change one element at a time. It gives you a chance to assess what really blends well with your personality.