usual relates to frequency, being customary
(stresses the
absence of strangeness; applicable to whatever is expected happens in the ordinary course of events)
common relates to being normal, not special
(implies the
lack of distinguishing or exceptional qualities; it
suggests usualness, everyday character or quality, or frequency of
occurence;
From:
Merriam-Webster's Dictionary of Synonyms. You should buy it.)
Try using this dictionary:
http://www.bartleby.com/61/29/C0512900.html
http://www.bartleby.com/61/88/U0158800.html