I read a very good article in a management magazine recently. Basically, they said that the mistake most companies make is in offering 2% (or whatever) for prompt payment.
Offering, say, 40% for prompt payment resulted in customers taking it seriously and trying to pay on time, as it looks worthwhile. Also a 40% premium makes it worth the time to chase the premium. Many companies offered, say, 2% just pay late and ignore the surcharge knowing no-one will go to court over that much. 40% makes them take notice.
Now that last bit might not apply to domestic customers, but it certainly would for a large commercial debt.
I'm sure 20% would have a similar effect.
Vin