Like Rebecca, I took a break from submissions over the holidays, and have just now gone back to my files to look at what pieces are where. I've got a ton of non-response entries, and I've noticed they've become more and more common as time goes on. I define a non-responder as someone who doesn't reply within the time they say they will, or who doesn't reply within six months for a short story or query. Though I have gotten responses up to two years after I wrote the person off as a non-responder.
Now if it's a short story market or someone who wanted to review something exclusively, I always chase those. I write a follow up email or letter, and if I still get no response, I send another communication to withdraw the piece from their market so that I can resume submitting. If it's an email query, which more and more agencies prefer, I assume that no response is a rejection because they no doubt get a million queries a month and just can't respond to all of them.
What do you do with the snail mail queries, in which you enclosed an SASE? I haven't been chasing these down, though perhaps I should. I only bother when the person says that I should in their guidelines. Those of you who have chased these, which method did you use? Another snail mail? Email?
My records are getting rather full of non-responders!