Should a writer ever be "in-between" projects?

For those of us who have not been published in our genre, like myself, I always wonder as to whether one should keep working on a project that is out on submission (i.e. the next book in the series), or if one should start something new, OR (God forbid) take a break and not write at all.

Part of me wants to keep working on the current project. The characters are fresh in my mind, their problems seemingly to mount with each day that I'm not at the keyboard.

Another part of me would like to revisit a previous project and maybe improve it.

Still, maybe it's time to think of something new to write.

Or maybe it's time for a writing break?

The issue with the last one is that the break can last much longer that I ever intend. Days, weeks accumulate into months, and that becomes a whole new problem in and of itself.

What do you do when you've typed The End and shipped the manuscript off to your agent - do you keep writing the same series, start something new, or take a break?