Have them sign promissory notes for next semester's dues at the end of the previous semester. It's probably too late for next semester, but ideally you would have them bound to their next semesters dues which helps you to a) adequately budget for the next semester, and b) send them to collections if they don't pay up.
Additionally, it sucks sometimes to be the bad guy, but its necessary. This article is about being chapter president, but I think it applies to the treasurer in many ways too:
http://fraternalthoughts.blogspot.co...your-turn.html