It depends on what kind of guy you are, my preference could be very different from another girls. I'm impressed when a guy feels comfortable in what he's in as long it's not dirty or overly tight wear what you want and impress me with who you are not what you wear.
clean dark-colored clothes,Tee-shirt with a cool logo on it maybe but nothing offensive, jeans (avoid shorts unless you're at the beach) maybe a nice zipper jacket or suit jacket on that. Maybe even a leather jacket, nothing too colorful though.
always suits for going out but never bowties! bowties are for little kid's, not grown men. Hope this helps!
It really does depend on the girl. In the winter I'm really into plaid long sleeve (preferably rolled up) shirts. PacSun has great shirts like that. I also like wife beater t-shirts and just a nice shirt with a fancy looking design on it (like ones you see from Buckle), with any kind of jacket, just not any bright colors for jackets. Nice fitting jeans, not holey.. I like both dark and light jeans.
In the summer, same goes for the shirts, and for shorts.. I like cargo and sometimes guys with black pinstripe shorts.. not plaid shorts.