I have a simple system
I send all my reminders once a week on a Sunday evening.
Everyone outstanding for 7 days or more gets a text.
Same every Sunday.
If a customer becomes due for their next clean with payment still outstanding they get skipped. (quite rare)
If the next clean rolls around and they still haven't paid (they have probably had half a dozen reminders by this point) they are removed from the round and told why and that if payment is not received in 14 days they will be passed on to debt collection.
These customers are then dumped regardless if they cough up.
One clean, one payment. This way you don't get taken for a mug, Ive been had for two, three and even four cleans before in the early days.
And yes don't panic, my rolling debt is £500 to £1500 regularly and the other week it was over 2 grand for the first time with two of us working. God knows how much some of the big boys must have rolling over each month.
I'm happy when I see mine drop below a grand and the only time I see it drop under £500 is if if I go away for a week or two.