My suggestion: Ban simple ‘post in this thread and/or friend me’ contests.

This might sound quite pointless, but I think that it would improve the quality of the contests nonetheless. I am aware that it would result in many contests which aren’t much better than the current ones – people would just get around the ban by doing contests with super-simple questions/requirements… but it would probably encourage people to put a little more thought into their contests anyway. And I would still prefer a “What did you eat for lunch?” contest over a “friend me and post ITT” one.