The most effective thing a small business can do about late payment is to stop carrying the whole value of the work until the end of it.
Deposits and staged invoicing are not aggressive. They are how most industries already operate, and customers who buy anything at all are used to them.
Deposits
Take one wherever the work commits you to cost before you are paid — materials, subcontractors, time you cannot recover, a slot in your calendar you are turning other work away for.
How much depends on what you are covering. Enough to fund the outlay, at minimum. Half is common for shorter jobs; a smaller proportion is normal where the work runs for months and is invoiced in stages anyway.
The deposit does two things. It funds the start, and it tells you something. A customer who will not commit anything to a piece of work is a customer who may not be committed to it, and finding that out before you have bought materials is considerably cheaper than after.
Milestones
On anything longer than a few weeks, invoice in stages against defined points rather than at the end.
Tie each stage to something visible and agreed — a phase completed, a delivery made, a sign-off given — not to a date on its own. A stage tied to a date invites an argument about whether the work justified it; a stage tied to a deliverable does not.
The effect on your cash position is substantial. Instead of carrying three months of cost and then waiting thirty days, you carry a few weeks at a time.
Payment before final delivery
Where the nature of the work allows it, arrange for the balance to be settled before the final handover — the files, the keys, the go-live, the last consignment.
This is normal in a great many trades and it is not distrustful. It simply means the leverage exists at the point where it is useful rather than after everything has been handed over and the only remaining option is to ask.
Agree it up front or not at all
All of this belongs in the quote. Introducing a deposit requirement after the customer believed the work was starting reads as a change of terms and damages the relationship, even where it is entirely reasonable.
Stated at the outset it is simply how you work, and in a year of quoting almost nobody will question it.
The one exception worth making
For a long-standing customer with a perfect payment record, easing terms is a legitimate commercial decision and worth making explicitly rather than by drift.
Just keep it deliberate, and revisit it if their payment behaviour changes. The customer who has always paid on time is exactly the one whose sudden slowness is worth noticing.
General guidance on commercial practice, not legal or financial advice.