I've got the top spin down, but can't seem to slice.
When the ball comes to your strong side (left for lefties, etc.), swing the wiimote with the buttons facing the same direction as your palm and as you connect with the ball, twist your wrist so the buttons face upwards. The spin should be very clear so you know if you did it or not.
I guess the same technique should work for backhand spins, but I can't manage doing them, unless I am doing them and they're just not as noticeable.