I only have one client that pays late consistently and I send them a reminder every two days after the invoice payment is due. They’ve never needed more than two reminders because I guess they know I’ll hound them and not take more work on if they don’t pay.
I’ve spoken to other freelancers who tell late payers there’s a strongly-worded letter from a lawyer coming their way if they don’t cough up and that can work, no really serious threats but just to scare them a bit.
I’ve not tried sticking a late payment charge on invoices yet, but it’s had varying degrees of success for people I know who do it. People who’ve had success with that generally put it in their T&Cs at the outset