Photograph by Toby Hudson, distributed under CC BY-SA 3.0.
It seems like common advice for unestablished novelists is that after you send your manuscript out, you should take a break from that novel and work on another project. It’s also advised that you should not begin writing a sequel in a series until the first book is picked up (supposedly, agents and publishers prefer a debut author’s first work to be a standalone so they need not commit to a series; of course, I’ve heard the opposite as well–people love them some trilogies).
Anyway, getting to me—’cause that’s really why I have this blog, to talk about me doing stuff—I’m considering doing all of the following at the same time: begin writing a sequel to my currently unagented manuscript; begin writing a new, unrelated novel; and continue editing my older manuscript and drafting an agent query for it.
Good idea or bad idea? I’ll think about it.
[Edit: I didn’t starting another project ’til a month or two after this post, and I only got about a quarter of a way through the first draft before I set it aside to start on another project that I’m much more likely to finish, referenced in this later post.]
A.D. Martin 
I definitely advocate writing the sequel before the first one is published. The last thing you want is to have inadvertently written yourself into a corner in book one, when changes prior to publication could have fixed the whole thing. Tolkien did that in The Hobbit, and it took some fancy footwork to get him out of his quandary (as the importance if the ring grew, the details surrounding it became more important; he turned the first printing edition into Bilbo’s version, claiming he lied about how he got the ring so Tolkien could change it and tell the true story).
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