Faculty Focus had a quick little blurb on getting students to do their reading assignments, outlining which standard techniques work best.
However, one technique they're missing: explaining what the reading will teach them and how the reading will aid them in life and/or work. Students don't read because the assignments lacks the weight of import. Especially with adult students, something often has to get sacrificed in education or life (there are only 24 hrs in the day) - reading assignments will fall off because students often don't buy into their importance. Help them understand the import.
A bonus of this activity, you'll be forced to evaluate the real importance of the reading. If it's not full, rich, and thick with weight, pitch it. Find one that is.