As a writer, how do you react to a bad review for one of your pieces?

This depends on the nature of badness!

If a criticism's focus is objective, I can make use of it and consider it as something useful. If it is a mere aggression that lacks objectivity, I will ignore it. I think positively and deal with issues as they are!

Pretty much miffed, initially, of course! But then I think of my readers and reviewers as my precious friends who take time to read and write a bad/good review. I'd delve deep into the reasons why I got a bad review. If the bad review is not justified, I would not enter into an argument..........I'd rather one of my readers answer that review. But certainly, reviewing is as much the right of a critic as mine is to write and expect him/her to read what I write:)-

Swallow it, and move on.

Well, I'd try not to take the critique personally; and might use it to improve myself.

I experienced that once.  My work was badly reviewed by an unqualified critique.  I wrote him back asking for the basis of his comments and pointing out one by one the issues he threw on me.  Realizing that I knew my craft, he apologized to me.  

Yes, if my work is badly reviewed by incompetent people, I asked for valid grounds.  However, if I see that the review is for my own good, I'd consider them.