...and by writing the next thing, right?
Well, not exactly. I'm not a very typical writer, and when I don't feel it, I don't do it.
Yep, the waiting stinks, but it's like juggling -- don't wait until you've caught the first ball you threw in the air before sending up the next
Believe me when I say riding my own tides without trying to direct them results in more than enough output; I finished another unrelated novel while I was still looking for agents, but after I got signed my flow was a little interrupted and I didn't really feel the creative thing happening except for a couple short stories.
In the meantime I focused more on blogging, art, drafting and finding separate representation for a nonfiction book (signed yesterday, yay!), and--what you're seeing here--connection with other writers. I've already got two finished novels in the bag to throw on the table if something happens with the fantasy series on submission, but I wouldn't dream of deliberately sitting on my butt waiting for something to happen. I just don't really feel like starting the second book in the series unless the first gets a deal.
I am a lots-of-balls-in-the-air kind of person, incidentally, and you're right that concentrating on the next project helps distract you from what's going on with the other one (I've given that advice myself!), but sometimes you just go through "connection" phases. I've been doing a mixture of that "connection" phase and the "consumption" phase (where I'm reading more books and reading about writing), because the "creation" phase doesn't seem to be hooked up to the fiction machine right now. Hope that makes a little sense (and hope I didn't unintentionally hijack this thread with my rambles).