Dear ________,

This letter is to inform you that your account is seriously past due and has reached a point where we must decide within a few days whether to turn it over for collection or just hand it over to our attorneys, neither of which is our preference.

I am sure you are aware of the manner in which accounts that have seriously past due balances must be handled. This letter is not to embarrass, intimidate or do any of the nasty things that are often done to encourage payment. 

We would simply like to hear from you and learn what your intentions are in regards to your account.  Please contact me immediately to resolve the past due issue of your account.