Mega-topics -- topics which have hundreds of pages (thousands of posts on one topic) -- should be avoided. Two things to keep in mind:
1. If you feel yourself going significantly off the rails of the original topic of a thread with a reply please
start a new topic instead of replying to that existing thread. If you must, you can post a reply to the original topic saying "Hey, I wanted to talk about X, so I've started a new topic (link here)"
2. If you see that you're replying to a topic that has over 50 to 100 pages on it, stop. Start a new topic, no matter what.
In theory, this will help out with the server load a little (fewer posts for the software to slog through in order to display the topic), so I'm not just asking this to be annoying.
With this message I'm also asking all board moderators to be on the lookout for mega-topics and to try to split them up into useful chunks -- even if it's just separating the old part of the topics off into another topic labeled something like 'Mega Topic Alpha [April through July 2007]' or something like that. When splitting off old chunks, please lock the old chunk so it doesn't take on any new posts.