How to charge late fees when clients don’t pay on time

This is easier said than done. Consulting work is mostly relationship based, and charging a late fee, even if it's contractually allowed, doesn't set a great tone.

I agree it's good to have the clause in there so you can enforce it if you've exhausted other options and are in a relationship straining conflict anyway. For normal business, if you've got customers who are slow paying (which are usually big companies of government that can't move quickly), it's better to just charge more up-front to reflect the cost of the inevitable delay