That is a good idea about giving them control over the music and costume--that would entice some people. What else can they offer? I don't think they can just throw money at it, but maybe they could do a few things, maybe...
Offer to donate to the star's charity a set dollar amount for every vote they receive. The longer they stay the show, the more votes they get, the more donated to charity.
Change the judges, or rules to make them more positive, less harsh.
How about a team-approach for Stars worried about the hard work--two Stars on a team, they dance every other dance. Of course to be fair all other couples would have to follow same rules. Once they reduce to half the pack they split up into individuals--at this point you figure anyone left is willing to put in the effort to win.