I require a deposit to book a date.  I request final Head Count 3-5 days before the event, I email the invoice 2 days ahead of the event and it must be paid in full 24 hours before the event date.

I always review my terms with potential clients before any events are booked to make sure they understand my terms.  I explain that I am a business and I run as such.  If someone gives me a hard time, or refuses to pay an invoice I remind them I am a business just like anyone else, and tell them that I can't deliver on their event without payment.