The easiest way that I know of is this: go to www.kedaman.com and download the kedatray ocx Kedaman has made. It works really good, and it's really simple to use. Then in the form_resize() sub add this code.
Code:
Private Sub Form_Resize()
If Form1.WindowState = vbMinimized Then
    Form1.Hide 'if you minimize it, it hides it
ElseIf Form1.WindowState = vbNormal Then
    Form1.Show 'shows the form when you bring it back
End If
End Sub

Private Sub Trayicon1_MouseDblClick(Button As Variant)
Form1.WindowState = vbNormal 'sets the window visible again
Form1.Show 'shows the form when you double click the icon in the tray
End Sub