A day or two before calling for a quote grab the garden hose and use it at full pressure to hose down all your windows, window frames and screens. That will get rid of most of the loose dirt, cobwebs and rinse your fly screens.
In most cases hosing down your windows will mean that you will not need your fly screens cleaned which will save you money.
Make sure you rinse your windows at least two days before your window cleaner is due to clean your windows because the seals can hold water for up to twenty four hours. If you rinse your windows to soon before your window cleaner is due the water in the seals will be running down the glass while they are cleaning the windows which will make the job longer and ultimately cost more.