Toma tio, sigue asi, sin pelearte con el código . . . .
Código:
Public Class Form1
Inherits System.Windows.Forms.Form
#CODIGO GENERADO
Private objFrmClientes As frmClientes
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
If objFrmClientes Is Nothing Then
objFrmClientes = New frmClientes
objFrmClientes.MdiParent = Me
objFrmClientes.Show()
End If
End Sub
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
If Not objFrmClientes Is Nothing Then
objFrmClientes.Close()
objFrmClientes = Nothing
End If
End Sub
Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Load
Me.WindowState = FormWindowState.Maximized
End Sub
End Class