I'm a big fan of Palmbobs' articles! And I'd like to contribute in some small measure for all the wonderful info he generously shares. Being a sculptor I routinely drill hard materials. Here's what works well for me.
Put the pot right side up on a scrap of plywood so that it's base is in contact with it and supported. Next build a small well or ring of clay (water clay or plasticine will both work well) and stick it to the bottom of the pot. Usually 1 inch high or less is all you need. Squirt a little water in to fill the well and begin drilling. Every few seconds lift the drill bit to allow the water to flow back into the hole. This will keep the drill site and bit cooler. Heat generated while drilling is the main reason pots crack and bits dull prematurely. It will also keep down dust (you don't want to breath silica dust). Squirt more water in as needed.
As well as a carbide tip bit which is great for softer materials such as terra cotta pots, a core drill can be used on the harder materials. I use it on granite and stoneware pots and it is quite a bit faster. This is a tube (available in various diameters) with diamond grit on it's rim. Again, a well of water facilitates cutting.