What should I do if I'm on the waitlist for a PhD program?
@Davidmh's advice is spot on, but I will definitely add that now is time to review your backup plan. (Of COURSE you have a backup plan, right?) There's always the possibility that you won't get into any of the schools you applied to; it sucks, but it happens. You should have a solid plan in place to make sure that your resume doesn't go stale in the interim. This could involve working as a lab tech/research assistant for a lab in your field, applying for internships in industry, taking on academic teaching positions, or even just finding any job to ensure you have income.

Whatever your plan is, now is a good time to start making sure it's feasible, and if it involves applying to jobs or the like, you should probably start doing that as well. You can always state that you cannot start working until after whatever the last date you can expect to hear from the university happens to be. The worst thing that can happen is that you have too many options. That is far better than the alternative.