Is Null Fields - Message Box

Does anyone know a nicer way of writing this. I basically want the form to quickly check all the fields to make sure they are filled in when the print button is clicked, different fields need to be filled in for different print buttons so i used this which works but seems very messy.
Private Sub Command0_Click()

If IsNull(reg) Then

MsgBox "You must enter Reg", vbCritical, "Data Required"

Else

If IsNull(make) Then

MsgBox "You must enter Make", vbCritical, "Data Required"

Else

If IsNull(model) Then

MsgBox "You must enter Model", vbCritical, "Data Required"

End If
End If

Exit Sub
End If



End Sub