What to do!

Okay, I'm sure many of you have this problem.  Working on my next novel, and ideas start popping in about the next book idea.  I write them down, but then its hard to get back to the current work.  I think I rushed through the ending of my first novel, to start this one, and now its happening again.  Do I put this away?  Do I try not to think about my next project?  What do you do?