When I had roommates I always just did the extra cleaning. The way I look at it is, the level of cleanliness you expect may not be the level they expect. People are raised differently / have different expectations, so what's acceptable varies. I've seen plenty of families that go beyond "not picking up" after themselves. They basically have a floor made of garbage with walkways they've created through it. Unless you set ground rules before they moved in, asking them to clean more than they want to is just imposing on their way of life.
Otherwise, try to get them to chip in for a cleaning service or something. Maybe they are just lazy.