An approach I've found useful is to do some of my private, $home projects in languages I'm not using for $work. Much of my Perl knowledge has been gained on such projects because Perl was not the main language at various companies where I've worked.

Much like any skill, I find that a little practice with the basics helps to keep the whole thing from slipping away. Obviously, some of the advanced stuff becomes rusty, but it does seem to come back when needed.