I just had to pop in here and say that my experience was similar to Jen's. I wrote LAMENT and really felt like it was the book that I was finally going to get published. I sent it out and while I was waiting, I wrote a mostly standalone sequel. By the time that I sold LAMENT and revised it with my editor (much to its benefit, I should add), the sequel was absolutely not relevant. There are still great ideas I want to plumb from that sequel, but there was absolutely no place for them in BALLAD, the real sequel to LAMENT that I ended up writing after the revisions for LAMENT.
So basically I wasted that time writing a fake sequel when I could've been brainstorming on a brand new idea. I learned my lesson and started a completely new project after LAMENT, which ended up selling within a few weeks of being sent out -- not something that would've happened if I'd started in on yet another sequel! I think writing a sequel before you're under contract is always going to be a dicier proposition that forcing your brain to start on something new. Plus a new project will give you objectivity on your characters from your first book and stretch your writing brain. There's a real danger when writing a sequel to your first book that you're just writing your own fanfiction -- that's what I was doing when I wrote my first fake sequel to LAMENT, back before it was under contract. :D
I always, always, always recommend starting something new.