It depends on the school. Some do the aforementioned process of reviewing you against the current months applicants and all the others on the list monthly. Others, like the UCs, are just super slow. I can't say whether being on the waitlist early helped me at UCSF, because I interviewed in November, and got in in late July. I don't know of anyone in my class who wasn't offered an initial acceptance who was admitted before May 15th (but there may be someone, I just don't know of them).
Make sure that if you are on a list of a school that you really like, that you let them know that. Write a letter to the admissions committee telling them how much you want to be a student there, and if you have any strong recommendations you haven't sent in yet, do so. Don't pester the school, but if you have any significant undertakings to discuss, or new reasons you want to go there, sending an update letter every month is advisable at some schools, okay at some, and too frequent at a few (ask current students what level of corrospondance is too much).
Good luck, and as stressful as it is to wait and not know, try to remind yourself that the year is still young.