How long should I wait before following up after being wait-listed? One of the faculty members of the Ph.D. program I applied to called me on the phone and told me that I've been wait-listed. He said "You're high on the wait list" and he said it's very likely that I'll get in, but people probably will start declining offers in March.

He said he'll be in touch with me, but I haven't still heard back from him after 3 weeks.

At this point, that program is my top choice among those that I still have a chance for. Should I follow up with him? (even though I still haven't received any offer that would be a reason to ask them to let me know the decision sooner.)

He told me "Your profile is strong and you probably have or will receive excellent offers from other places, so we understand if you prefer to take them", which (besides adding to the pain of actually receiving no offers), makes me worried that they might guess that I have other offers and won't wait for them, if I don't contact them and express that I am still very interested in their program.
makes me worried that they might guess that I have other offers and won't wait for them, if I don't contact them and express that I am still very interested in their program.


I wouldn't worry about that.  The fact that he called you on the phone to me indicates that he really would like to make an offer to you, but he can't guarantee it now (hence the "so we understand if you prefer to take them").  Unless you told him something to indicate that you wouldn't be interested in an offer later, they will not pass over you as the move down the waitlist (of course, there's no guarantee they will move down the waitlist, but it sounds promising).

Since he said people will probably start declining offers in March, he probably won't have any updates on your application status until sometime in March, so it seems pointless to contact him before March, particularly if you do not have another offer you need to decide upon yet.  I understand being anxious, but take a deep breath, and try to be patient.  There's no need to contact him again until you have some decision to make (or if take another offer, so you can withdraw your application), certainly not before mid-late March.