protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        string prevent = "if (typeof(Page_ClientValidate) == 'function') { " + 
                         "if (Page_ClientValidate() == false) { return false; }} " + 
                         "this.value = 'Submitting...';" +
                         "this.disabled = true;" + this.Page.GetPostBackEventReference(ButtonSubmit) + ";";
        ButtonSubmit.Attributes.Add("onclick", prevent);
    }
}