Name recognition is the most powerful element in your subject line.
If you were to sign up today for a Joe's Books newsletter, the best way to catch your attention in your inbox tomorrow would be to include the word "Joe's Books" in the subject line.

You will immediately think something along the lines of, "Sure, I signed up to hear from these guys. I’ll open this one."

So while you only have about 40 characters or so to use in your subject line, find a way to include the name of your site, your newsletter or your editor in the subject line... whichever you feel will create the strongest name recognition.