What should I do if I'm on the waitlist for a PhD program? I'm on the waitlist of a top 2 program in my field which has a good track record of accepting waitlisted applicants. They told me to wait until April 15 and then I would hear from them. Interestingly, I got rejected by every other program I applied to, so I have no other place to go to. This is my second admissions season - I applied to PhD programs last year as well, got waitlisted from a couple top programs but it didn't work out.

I'm going to give up after this season if this waitlist doesn't work out. Is there anything I can do to maximize my chances of being admitted off the waitlist? 

I don't want to seem obnoxious when I email the director of graduate studies at the university, so I don't want to talk about how I don't have any other plans or something like that, but what else can I do?
@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.