protected override void SetVisibleCore(bool )
{
   base.SetVisibleCore(false);
}

static void Main()
{
  Application.EnableVisualStyles();
  Application.SetCompatibleTextRenderingDefault(false);
  System.Resources.ResourceManager resources =
    new System.Resources.ResourceManager("myResource", System.Reflection.Assembly.GetExecutingAssembly());
  NotifyIcon ni = new NotifyIcon();
  ni.BalloonTipIcon = System.Windows.Forms.ToolTipIcon.Warning;
  ni.BalloonTipText = "test!";
  ni.BalloonTipTitle = "test.";
  //ni.ContextMenuStrip = contextMenu;
  ni.Icon = ((System.Drawing.Icon)(resources.GetObject("ni.Icon")));
  ni.Text = "Test";
  ni.Visible = true;
  ni.MouseClick += delegate(object sender, MouseEventArgs e)
  {
    ni.ShowBalloonTip(0);
  };
  Application.Run();
}

private bool windowCreate=true;
...
protected override void OnActivated(EventArgs e)
{
  if (windowCreate)
  {
    base.Visible = false;
    windowCreate = false;
  }
  base.OnActivated(e);
}
private void notifyIcon1_DoubleClick(object sender, EventArgs e)
{
  if (this.Visible == true)
  {
    this.Hide();
    this.ShowInTaskbar = false;
  }
  else
  {
    this.Visible = true;
    this.ShowInTaskbar = true;
    this.WindowState = FormWindowState.Normal;
    //this.Show();
    this.BringToFront();
  }
}