private void createCookie(string username, int customerID, bool persist)
{
    HttpCookie cookie = FormsAuthentication.GetAuthCookie(username, persist);
    cookie.Expires = DateTime.Now.AddHours(12);
    var ticket = FormsAuthentication.Decrypt(cookie.Value);
    var userData = customerID.ToString();
    var newTicket = new FormsAuthenticationTicket(ticket.Version, ticket.Name, ticket.IssueDate, ticket.Expiration, ticket.IsPersistent, userData);
    cookie.Value = FormsAuthentication.Encrypt(newTicket);
    Response.Cookies.Add(cookie);
}

protected void Login1_LoggedIn(object sender, EventArgs e)
    {
        //... unimportant code left out

        //Update the users ticket with custom userInfo object
        string userData = userInfo.Id.ToString("N");
        HttpCookie cookie = Response.Cookies.Get(FormsAuthentication.FormsCookieName);
        FormsAuthenticationTicket oldTicket = FormsAuthentication.Decrypt(cookie.Value);
        FormsAuthenticationTicket newTicket =
            new FormsAuthenticationTicket(
                oldTicket.Version,
                oldTicket.Name,
                oldTicket.IssueDate,
                oldTicket.Expiration,
                oldTicket.IsPersistent,
                userData,
                oldTicket.CookiePath);
        cookie.Value = FormsAuthentication.Encrypt(newTicket);
 }