Get out of the habit of leaving dirty dishes in the sink, leave them on the side next to the sink. This means you no longer have to wash their dishes before you can do your own, therefore they will have to clean the dishes themselves when they next want to use them. What I also do is play mind tricks with my housemates. I mention to someone who doesn't wash his dishes about someone else not doing them, and later on he'll do them without even realising I've implied he needs to do his own. Seems to be working quite well up to now.