one thing i\'ve found to work quite well, apart from watering the paints, is to take the paint straight out of the pot, and then gently touch your tongue with the brush. This thins the paint enough to be able to put down a smooth, slightly transparant layer, while still being thick enough to avoid the chalkiness (which i think is caused because the pigment is not ground fine enough and/or not distributed evenly enough troughout the paint)