Hey, maybe this would work. Say, after, say, a full month with no postings for a thread, there could be a message displayed somewhere in that thread, maybe prominently, that simply gives the number of days or months since the previous posting. Make it harder to miss the fact that the thread is more than simply resting.
I second that. It has happened to me quite often to see a thread, have something to add to it, write a reply, and notice only afterwards that it was a really old thread. I very rarely think of checking the dates (in my defense, I come to the boards with an interest for ideas, not calendars), and I was actually amazed to see how offended some people get when it comes to zombie threads being resurrected.