To be simple, we would use 'common' to refer to something that is numerous, while 'usual' would refer to something that is regular.
He usually goes to Church on Sundays.
Commonly, people go to Church on Sundays.
In the first, we are discussing this man's routine, in the second we are discussing the number/percentage of people that partake in the activity.
With that in mind, we would say
The robin is common to all parts of England -not- The robin is usual...
He ordered his usual at the bar -not- He ordered his common...
Where these words are used as adjectives to describe people, there are further differences.
Usual as an adjective for a person is normally positive and means that this person is not out of the ordinary; they are not 'strange' or 'weird'
Common as an adjective for a person is normally negative and means that that person is of the working class, taken from a time when caste systems were rigid and important. Therefore, a common person is one who does not observe finer culture - because they are uneducated or else cannot afford to.
Hope this helps.