Nothing spectacular... but I've gotten into the habit of storing
away my current shells' command histories whenever I feel I've done
something non-trivial (debugging/compile session, etc.). For that I
have a little script which essentially does history >~/memos/YYYY-MM-DD_hh-mm-ss.hist
(YYYY-... being the current timestamp). Occasionally, I clean them up a
little, or add a few short notes, but more often I don't.
I can then easily grep through those files whenever I can no longer
remember what I did when and how... Also, I can
cut-n-paste stuff (can you remember all the hostnames and portnumbers
you typed two months ago when you had set up that three-hop SSH tunnel
to some client? — well, I can't). The possibility of
chronological access (kind of like a 'diary') has sometimes turned out
useful too, to reconstruct things — when someone asked or came up
with strange claims...
All in all, this has proven very useful, considering that it takes
almost no extra effort once you've made it a habit... (which is
important for the generally lazy chick that I am :)