Hi Emily.  Our company keeps a master aging spreasheet with notes as to what action has been taken to collect.  After many years of dealing with a lot of the same usual suspects you get to know what needs to be done.  Fax is fast and cheap but not a lot of people use it anymore.  Email is not always reliable, people and addresses can change often.  When in doubt we snail mail.  It's slow, tedious and costs money but you know it's harder for them to ignore an invoice copy stamped "Past Due" in bright red ink.   I leave the delicate task of phoning/personal visits to the sales reps who call on the accounts.   They are most likely to get to the right person without killing the account.  Fun fun!