Stay familiar with the tools you use most often (for me, my shell, my editor, Perl, C, LaTeX, Solaris). Even after more than a dozen years of intensive usage, I still reread the documentation of my editor every know and then (once every 1 to 2 years). Not to look something up, no, from top to bottom. Sometimes, I pick a Perl manual, and reread, top to bottom. Even if I know it already. I can advice that to everyone (and if you do, send a patch for every typo, grammar mistake, omission, wrong example, whatever, you find - you're much more likely to spot them then, then when you're browsing it for a particular piece of info). Other manuals (or books) I've reread multiple times, just for the sheer joy of rereading include the bash manual, K&R, and The UNIX Programming Environment.