how can stop by drop down lists from populating on every selected chang event?

how can  stop by drop down lists from populating on every page load event?  This is driving me crazy.  Need help, please.  I have five drop downs that cascade from one another. The problem I am having is when I go back to a previous drop down list and make a selection it repopulates the next one and ammends more data to it. can somoene tell me how I can caascade these drop down and have them populate without it duplicating the items.  do I have to clear the dropdown list/ if so , how??  thanks

my code...

  Partial Class Player_picks
Inherits System.Web.UI.Page

'declare varaibles
Dim pick1 As String
Dim pick2 As String
Dim pick3 As String
Dim pick4 As String
Dim pick5 ...