I have-
on quotes and invoices "Payment terms (unless previously agreed) are 7 days from invoice date, Any costs incurred in collecting overdue accounts will be passed to the customer, interest will also be charged on overdue accounts"
I send out one reminder if necessary, with a copy of the terms & conditions again, then I get a payment!, but I live in an area where word gets around, if someone is a bad payer, folks get to know, only been taken twice in 18 years, for a grand total of about £600, both "customers" finished up regretting messing me about, one had hell of a game getting work done after, and still does 6 years on!!