It's a humid and dusty area.
It's not just restricted to my arduino parts, but, for example, the sensor shield I had, the connector pins becomes dull grey in color after a few weeks.
Same for the usb port on my arduino.
For my desktop machine, the exposed usb port is even worse and become rusty. 
I generally can clean the external of the usb port a little using autosol as mentioned.
But I wish I can restore the shine on the connector pins on my sensor shield, contact point on the usb and the likes.
Thanks.