You can offer a block of 10 hours, and give a 10% discount for pre-paying for service. Also, be honest. Tell them you are trying to make it a win-win. You need the cash flow and value your relationship with them, and they need the savings they think they're getting by delaying payment. Tell them it's your new business model. Offer the same block of hours to other existing clients. You can either expire the block of hours' balance at the end of the year, carry over a portion (5 hours), or it never expires...your choice. I definitely feel your pain, and the position you're in - hopefully this approach helps!