I'm in the same boat (with the exception that I hold 0 acceptances). The only advice I received, and that I'm able to pass on, is that it depends on the school. Some schools arre open to waitlist updates, other schools are not. So far, the waitlists I'm on make it very obvious where they stand on the "to update or not to update" fence.
As long as the school accepts updates, find something (anything!) to update them with. Keep it short and to the point. You know it's not substantial, they know it's not substantial, don't waste anyone's time. But you still want your name to get seen again. Then around April 30th (the new acceptance decision date) send in your letter of intent/interest. Obviously if it's a ranked waitlist this means nothing as I don't believe you can influence your rank. If you will absolutely attend a waitlist school if accepted, let them know that in this intent/interest letter. They know you like them, so give a compelling reason as to why they need you/why you would be good at their school. They will pay little attention to the updates prior to the LOI, but imho it is only advantegeous to show your continued interest (assuming they welcome updates). If there is nothing revolutionary to update with, I would think it'd be better to show sustained interest than send nothing and hope for the best.