Removing duplicates

Greetings,

I have a customer database (name, address1, address2, city, state, zip) and 
I need a query (or two) that will give me a mailing list with the least 
amount of duplicates possible.  I know that precise matching is not 
possible, i.e. "P.O. Box 123" will never match "PO Box 123" without some 
data massaging, but if I can isolate even 50% of any duplicates, that would 
help greatly.

Also, any suggestions on which parameters to check the duplicates for?  My 
first thoughts were to make sure there were no two addresses the same in 
the same zip code.  Any insight (or examples) would be greatly appreciated.

Thank you,
Matthew