Good practice about DropDownList Databinding

Here is how one should bind the drop-downlist...and why is it good?
Because, in most cases we are executing some logic like loading another drop-down list on change of selected index of the first one.

Make functions to bind respective drop-down lists. And call them when needed to re-bind.

Page_Load
{
BindFirstDropDown();
FirstDropDown.Items.Insert(0,new ListItem("Select", "0"));
if(SecondDropDown.Items.Count == 0)
SecondDropDown.Items.Add(new ListItem("Select", "0"));
}

FirstDropDown_IndexChanged(...)
{
BindSecondDropDown();
SecondDropDown.Items.Insert(0,new ListItem("Select", "0"));

}
void BindFirstDropDown()
{
...
}

void BindSecondDropDown()
{
...
}