One thing which I think would be a really handy visual for necro'd threads would be a different background color on posts that are greater than say 3 months old.
Or a thick red horizontal line separating the old posts in the thread from the new ones.
The way some boards handle it is by having a warning pop up when you go to post a reply - similar to the warning we have when new replies have been added while you're writing a response. It's useful because people often don't look at the date of a thread when they've found it via search and the information contained therein could be hopelessly outdated. Warning people that it's an old thread before they can post a reply brings this to their attention.
Many forums have a policy against duplicate threads, in fact. Prohibiting necros is equivalent to adding a policy requiring duplicate threads.
The policy is often against having more than one active thread on a topic in a sub-forum, and there are times when it would be useful around here as useful information often gets lost when it's posted across 10 different threads on some hot button issue.
On the whole, though, this place isn't really busy enough to require dedicated mega-threads, and the reality is that when they exist people rarely bother reading beyond the first page or two so the same questions, answers and observations get posted over and over anyway.
Poll
