Since I'm sure you hate to turn down business (as tempting as it is), here's what I'd do. This winter, when she requests your services, I'd say, "Certainly. I'd love to help you out. Now this winter, in an effort to protect my business a bit, I'm asking all of my customers to sign this agreement before I begin working for them. If you'll just sign that, I'll get started." Then, provide her with a copy of the following:
I __(print name)___ request the snow removal services of KoKo. I will be billed at a rate of $10/hour for KoKo's services, to be paid immediately upon the completion of service. I understand that KoKo will use whatever snow removal method that she deems appropriate in clearing my drive. Likewise, I understand that, while KoKo will take the utmost care of my property, KoKo is not liable for any damage to my property done in the normal course of her business.