I have only had one customer pay late, and i just kept sending them the freindly reminder letter every 2 days (just to let you know that you have 1 outstanding invoice for $xxx.xx) and i send a copy of the invoice with a giant "Overdue" written in a giant red sharpy marker on it, and suddenly i got a check in the mail. wha who.....
i hate having to call people to collect money, just makes me mad.