Tracy Marchini (an assistant at Curtis Brown) had a good blog post this week about when and how to follow up on a query or partial that's been out there for a while. Here's the link:
http://www.tracymarchini.com/2009/03/how-and-when-to-follow-up-with-agents.html
http://www.tracymarchini.com/2009/03/how-and-when-to-follow-up-with-agents.html