You will always have the client that's really good at his trade but not at business so you have to work with them. So I first  provide them with options like monthly withdrawals or a 2 payment or 3payment option using their credit card or from their bank directly.
 If that doesn't work  because we always send them their renewal 45 days ahead we also send the invoice with a notes of late payment charges after 30 day of renewal date. If that doesn't work then we explain that we are not a good fit so they should look for a new broker doing this usually beings them around if they like you services as they know that other broker won't possibly work with them in the same way and they become a better client or move on and give someone else the problem.