As a player, I've taken to always keeping a campaign diary. During our last long-running campaign (about 2 years before the DM burnt out), I found that we were having a terrible time remembering the details from events more than a couple months ago. We didn't really have any form of record-keeping, and as the campaign got longer and more involved, we started seriously regretting it. At one point, we had to introduce a new player and summarize the campaign world and the previous year's events for him -- that was a tough one.
In the campaign I'm currently playing in, I'm keeping a log of every session, usually transcribed from memory or from shoddy notes the day after the game. It's too hard to keep good notes and stay in character at the same time, but I seem to have a decent short-term memory for conversations. If I were DMing, I'd probably have to tape the sessions, though.
I can't recommend keeping a detailed log strongly enough. It settles a lot of "when did that happen?" confusion and keeps the DM from forgetting the minor friends and foes which make a campaign memorable.
