Invoice immediately rather than at month end. A business invoicing weekly instead of monthly halves its average collection period without any change in customer behaviour, because the clock starts sooner. This is the single cheapest improvement available and it requires nothing from anybody else.
Make paying trivially easy. A payment link rather than bank details, correct reference numbers, and whatever specific information the client's system requires. A meaningful share of late payment is friction rather than reluctance, and asking a new business client what their invoice needs to contain takes one message and prevents a month of silence while their system rejects your paperwork.
Set terms deliberately rather than accepting a convention. Thirty days is customary and not obligatory, and shorter terms are frequently accepted without comment if stated at the outset. Payment due on receipt is normal for smaller work. What you cannot do is shorten them retrospectively, which is why they belong in the agreement rather than in the invoice.
Follow up on a schedule rather than when you notice. A reminder a few days before the due date, one on the day, one at a week overdue, and a phone call at two weeks. Automating the first three costs nothing and removes both the memory burden and the awkwardness, because a scheduled reminder does not feel like an accusation.
Remove the category entirely where you can. Deposits on project work, automatic payment for anything recurring, and payment at the point of delivery for shorter jobs all eliminate the collection question rather than improving it. For a business with predictable work, moving customers to automatic payment is the single change with the largest effect.
Then apply your terms consistently, including late fees if you state them. Terms that exist and are never enforced train customers that the dates are decorative, and the client who pays at day sixty is frequently doing so because nothing happened at day forty five.
Track how long each customer actually takes rather than how long they are supposed to, because that record tells you who to require deposits from and who can be trusted on terms. It also makes your cash forecast accurate, since using contractual dates for a client who reliably pays at day forty five produces a forecast that reassures you and is wrong.