Yo alexis,

as happy as a sandboy is one of my favourites - a very common English/British phrase (apparently led to some confusion in the US when used in Bridget Jones's Diary**). I recently discovered the origin of the phrase. Thought I'd already posted the link on AWAD but can't find it now, so here it is again:
http://www.briggs13.fsnet.co.uk/book/s.htm

Capital Kiwi referred to another of my favourites, in a thread containing lots of happiness :
http://wordsmith.org/board/showthreaded.pl?Cat=&Board=miscellany&Number=68894

Oh, and yet another favourite (is this used anywhere other than England?):
as camp as a row of tents

And here's a good one for rumination:
as thick as thieves
I checked and "thick" has no less than 6 meanings. This phrase uses one of the least common, I think.

Fisk

** Though not as much as "I'm dying for a fag"