How to effectively manage source control for project with both open and closed source?

Question

I'm currently working on a project that has both an open-source "community edition" and a set of closed-source features for paying clients. One of the pain-points right now is figuring out how to handle keeping the shared source in sync between projects.

We use Mercurial for source control, and the open-source piece is pushed to both CodePlex and Kiln, while the closed-source piece is pushed only to Kiln. We're presently keeping them in separate repositories with project references into the open-source repository where applicable.

Is this really the best way to handle this kind of situation, or if there's something I'm missing (like using a subrepository within the closed-source repository to contain the open-source portion) that might be easier and cleaner to work with?