The blog for the BookEnds Literary Agency has an interesting discussion going on right now titled "What to Write Next". Jessica, one of their agents, says if you have a novel going through the submissions process, you should work on an unrelated novel, because if you end up having trouble selling the first, you certainly won't be able to sell its sequal.
I agree. Much as I'd like to write the second book in The Timeless Empire series (the first has been making rounds for three years now) I won't. Well, in moments of weakness I do work on it, and in fact I've had enough moments of weakness that I'm a third of the way through it, but I've had enough moments of strength to write and submit another novel (fingers crossed) and get halfway through a third.
I still haven't sold my first novel, but now I'm shopping around another book too, instead of hoping the first will sell so I can sell its sequel.
It's important to keep forward momentum when you're a writer. I like to work on a few projects at a time and have lots of proposals being considered by agents and editors. As you pace the floor waiting for your dream agent to call, carry a notebook so you can work on your next book.