SharePoint WebPart having dropdown with indexchange event

In WebPart which is having dropdown,getting dropdown selected value on index change event is always a difficult task because whenever we change dropdown index page referesh and WebPart Createchildcontrol methos gets call. Because of which dropdown selected value gets lost.
Below is the code for getting dropdown selected value on index change

public class webservicewebpart: WebPart
{
Label lbl=new Label();
DropDownList ddl=new DropDownList();
protected override void CreateChildControls()
{


ListItem lstItem = new ListItem();
lstItem.Text = "One";
lstItem.Value = "One";
ddl.Items.Add(lstItem);
lstItem = new ListItem();
lstItem.Text = "Two";
lstItem.Value = "Two";
ddl.Items.Add(lstItem);

ddl.AutoPostBack = true;
this.Controls.Add(ddl);
// lbl=new Label();
this.Controls.Add(lbl);
ddl.SelectedIndexChanged += new EventHandler(ddl_SelectedIndexChanged);
}

void ddl_SelectedIndexChanged(object sender, EventArgs e)
{
//throw new Exception("The method or operation is not implemented.");
lbl.Text = ddl.SelectedValue;
}


}