They get turned over to collections (Tier 1) at 60 days, not 90. A "10 Day" letter prior to collections usually gets payment quickly.
Our protocol is to run statements weekly for anything over 15 days. It is clear that the Invoice is left on the door and is due upon receipt. At 20 days, the customer gets a personal phone call.
I have had a few problems from career deadbeats, but overall, my A/Rs have been great.
And for the occasional hot check, a letter from the District Attorneys office usually educates them.