Here is another thought. Offer a seasonal package price. They get the drive way power washed and sealed in the spring. You come check it in the summer and clean any oil spots or just a touch up if needed. And in the fall power wash again so it looks good for winter. If it needs to be resealed, not likely if you use a quality product, then that would be extra. Be sure to charge more initially to cover your return visits. And when you are there, that is a perfect time to inform them of your other services. Once a person trusts you they are more likely to purchase from you again.