Like most Brassicas, Kale likes cool nights (45F) and temperate days (60F - 70F). Sustained temperatures above 75F will cause the leaf of Brassicas and Lettuces in general to take on a bitter flavor. Cool them back down for a week (while still growing) and the bitterness goes away.
I like to grow Kale in a 10-inch pot so that it gets some serious roots and hence produces a lot of foliage. It doesn't require much fertilizer, consider a 5-1-1 Fish Emulsion or 20-5-5 water-soluble with micronutrients. If you are growing Lacinato, then I also recommend you try a pot each of Silver Rib Kale -- also sold as Winter Bor, and Red Kale (Bor).
