I'm sure I'm not the only one who finds necroposting annoying, so I'd like to make a suggestion. Whenever someone tries to post in a topic that is older than, say, two or three months, a big ol' message box should assault their screen saying something like this:
WARNING: This topic is 3 months old. People typically don't like necro-posting unless there is good reason. Are you sure you want to post in this topic?
Please don't tell me we already have something like this, and people just click "OK" and ignore it.