Unless you did really well or have been doing something new, don't bother. When I applied, I sent one update to a school that hadn't sent its decision yet. I'd gotten a 4.0 that fall, so I sent an e-mail saying that I was still very interested in the school, and here's an update on what I've been doing.
If you didn't do so well, you could just send an e-mail saying you're still very interested.