I've not done it personally, but everything I've read says to cut or drill clay/ceramic pots you need to soak them in water to get them thoroughly waterlogged and then keep them wet while cutting/drilling, otherwise you will just destroy them.
it may be necessary to soak, cut/drill a little bit until you see it starting to dry out, soak it some more, continue to cut/drill until it starts to dry, etc.... again though, I've not done it so I'm just being a parrot here. I tend to just smash the pot and see what I end up with *shrugs*
-Dave