What can I do to keep my cat from chewing on real and artificial houseplants?

Check it out at a pet shop, green and white bottle of spray called Bitter Green Apple. It also stopped my child from thumb sucking.

And you may want to consider growing kitty some safe plants of his own to nibble on. You can find grass kits at any pet store, or just pick some up at a gardening supply store. Then, you just need to redirect him from the "bad" plants to the "good" ones.

And keep lilies out of your house until you have this under control!
51. Easter lily (http://www.vet.purdue.edu/depts/addl/toxic/plant51.htm)
CFA: Lilies Can Be Deadly to Your Cat (http://www.cfainc.org/articles/lilies.html)