This is the first of my tutorials on various useful crafting techniques. Sometimes I will do a tutorial myself, sometimes I will just link to tutorials elsewhere.
How to paint glazed ceramic pieces - Get some fine grit sandpaper and
gently rough up the glazed surface. Then spray the item with Krylon porcelain finish. Then you can paint it with regular paints. After the paints are really dry, spray or paint on a clear sealer. Use a matte sealer for a flat finish, and glossy for a shiny ceramic look finish. When using spray paint or sealer, several very light coats work better than heavier coats, which get runs and drips in them. Here’s some more tutorials on painting over already glazed ceramics:
Spray Painting Ceramics Tutorial