I'm looking for a contributor, but I also know people are flakey... Especially the group I hoped to work with (college students). I intend to do my share, how

I'm looking for a contributor, but I also know people are flakey... Especially the group I hoped to work with (college students). I intend to do my share, how do I ensure a contributor does theirs and not make my life hell if we decide to put ads on the page after they disappeared several months prior?