Retrieve Data once

Hi
I have a query 2 Querys for my reports
Retrieve All Department Retrieve Department Manager at Page load of the Report page I need to Query the All Department and bind it to my dropdownlist
Once the user Select a Department Query the Department Manager.

Now, What I want to happen is to Retrieve All Department with their respective Manager instead of retrieving the Department Manager in every Selected Department.
But I don't have any Idea on how to apply it. 
I tried declaring a datatable as public on my Reports page but in everytime the Report page postback it just clear the value of my datatable.

Thanks,

Ron