BigMack
In addressing your present issue -- obviously you can't do all the jobs at once. I assume you schedule them on the basis of when the order is received? Do you give the customer a specific date (or date range) on which work on their project will begin? If you do this, I assume you give yourself a little cushion for bad weather etc?
The most important thing is communication. If you tell me you're going to do my parking lot beginning on August 5th, then call me on July 29th and tell me you are going to begi on 7/31, I will be happy. If you have to push the start date back due to weather, I will be less happy, but at least I am informed. I suggest giving a date, then let them know you will follow up a few days ahead of time to confirm the start date for the project. The most important thing is that the customer be kept informed of the status of their project.
You could also consider charging a premium to get a project "bumped up" the list for early completion...