By the hour!!! There are too many variables especially for leaf clean-up. One day the leaves will all be completely dry, you give them a price, you go to do the job a week later, it has rained for 3 days, now the leaves are soaking wet. Now the job will take at least double the time, but you already gave a price for how they looked at the time of the estimate. You only get paid for half of what the job should pay. I learned the hard way. Customers dont always like to pay by the hour, but usually if you explain this to them they completely understand.