Card sorting - too superficial?

Donna Maurer muses on a card sorting weakness - In my last few big card sorts, I have noticed that participants don't really look at the cards and try to form sensible groups based on how the information should be grouped to help them achieve a task. Instead, they try to get rid of most of the cards as quickly as possible....

Good point: almost all participatory research, from card sorting to usability testing to surveys and interviews suffer from the fact that the participants usually don't really want to participate. While our goal is to build a better system, their goal is to finish and get their incentive.

Other than longer education periods and involving users as codesigners (so they are invested in the project itself, not just the incentive), what other ways are there to get more than surface data from a card sort? One immediate thought is to make sure the card set isn't too big (or the usability script too long), since people are more likely to give cursory answers when they've spent a long time on the activity. What else?