I'm sure that many of you have also worked this little tip out yourselves, I came up with it a few years ago, but having just given our Shogun a very full valet prior to selling, it never fails to impress me.
It involves a short length of hosepipe and a clump of masking tape. Chop off about 18" of hosepipe, wind around enough masking tape about to form a clump big enough to plug the end of your Hoover/wetvax, about 4" from one end of the hosepipe. Shove the short end in the Hoover, wriggle the clump of tape to form a seal and off you go. It'll get into all those awkward crannies and concentrate the suck.