I'd dip it in a tub full of lye and mek; that mixture will dissolve all manner of
evidence grime and gunk. Hell, it'll even dissolve the case!
On second thought, that may be a bit too aggressive for you. If it's just a bit dirty and dusty, a soft cloth and a soft bristled brush (to get into the nooks and crannys) with a little multipurpose cleaner should do the trick. If it's yellowed, I advise you to read up on 'retrobright' cleaning (click
here and
here).