I'm often asked what makes the "perfect client," and the only answer I have is a great sense of communication. The desire to keep me posted on all things good and bad so that if something can be done (even if you don't know it can) I can jump in and do it.
One thing a few of my clients do that I LOVE is send me monthly or quarterly updates. Even if it's information I know or have it's extremely helpful. I get an email that says something like this:
Delivered Book 3 in series X, awaiting revisions
Finished copyedits on Book 2 in series Y, love the new cover
Starting promotion on series Y
Release date of series x is next week
Had long talk with editor about new ideas and will run those by you when I have time to think about them more.
Now, I do try to check in with clients occasionally, and to some maybe I check in too often, but an email like this helps the author center herself and feel a sense of completion and helps the agent step in with thoughts she might have. Maybe I never saw the cover so can remind her to send me a copy for the website, or maybe I just had my own brilliant idea for a new idea for her and this is the perfect time to send it along. Even if I have nothing to add, keeping an agent in the loop helps everyone work together better, which is what we're all striving for.