Use an electronic cashbook for your bookkeeping. It makes GST a doddle and can save oodles of time at the end of the financial year.
If you don't really believe that one of your slow paying debtors will keep to the payment arrangement you both agree to, get that arrangement in writing. Once you do, if he defaults on that arrangement and refuses to pay any more, you can litigate against that slow paying debtor and it seems you cannot fail to win your case.
That's because you can take him to court on the new contract, the payment arrangement. By signing that arrangement, the debtor is signing a new 'contract' with you, so, you tackle him on the broken arrangement. He can't argue about fees raised for your charges, because you're not taking him on about them, you're taking him on about the broken payment arrangement!
Just three weeks ago, this happened ...
The Debtor - Mr Jones
The Creditor - An Accountant, Matt
The Debt - $20,000 remaining
Creditor: "But you agreed to pay $5,000 a month Mr Jones."
Mr Jones: "Look, I'm just not happy with the work that Matt did. He charged far too much for what was done. I want a full breakdown of his charges before I'll pay anymore."
Long story short - we took him to court (on the broken written arrangement).
We won.
And here's food for thought:
Q: What if the debtor refuses to sign a written agreement?
A: What does that tell you about his genuine intent (or lack thereof) to pay?