protected void Page_Load(object sender, EventArgs e)
{
    // clear collapsed child nodes every postback
    if (IsPostBack && !IsCallback)
    {
        FlyTreeNodeCollection collapsedNodes =
            FlyTreeView1.FindAll(delegate(FlyTreeNode node)
            {
                return
                    node.Expanded == false &&
                    node.PopulateNodesOnDemand == false;
            });
        foreach (FlyTreeNode node in collapsedNodes)
        {
            // clear child nodes
            node.ChildNodes.Clear();
            // and make them populate on demand again
            node.PopulateNodesOnDemand = true;
        }
    }
}