Another option to consider is splitting the gross profit with the customer. Say that your gross profits (revenue minus the cost of servicing the property, which includes labor, gas, etc.) is about 50%. You might decide to offer your customer half of that profit (25%), in exchange for them paying you early. You shouldn’t go beyond this because you need the other half of your profit to cover your overhead costs (rent, insurance etc.).
While a 25% discount is significant, it could be justified when you consider the cost of furloughing and having to rehire new workers come spring time. Furthermore, when you consider the benefit of locking in customers for the following year, and freeing your time to get additional customers, it could be worth the expense.