My favorite around the hospital is “going home with a capital H”.
That was also my understanding – you bought a grass farm, 8’ x 2’, with head stone.
A personal favorite that’s under used: paws up.
Isn’t this one for dying, and not actually dead? Someone who’s very sick could be called “circling the drain,” no?
Yup.
These are all well and good but as yet no-one has explained why "Kick the Bucket is used to mean somebody has “Cashed in their Chips”
I dunno about the bucket one, but I bet the chip thing has something to do with British cuisine.
Snap! 
I just had a thought.
Maybe the bucket one refers to Mrs Whatevers cow, yannow the one that caused Bostons fire.
The chips could be a casino reference, nuffink at all to do with our fine food
Shit the Bed
Never trust a fart?
Kicked (or gave up) the oxygen habit.
The most plausible explanation for kick the bucket is that “bucket” is an archaic term for a beam from which animals were hung by their feet to be slaughtered. The animal would kick its feet in its death throes, and so, quite literally, “kicked the bucket.”
oft-used by Rush Limbaugh: “assumed room temperature”
Went to the Happy Hunting Grounds
gangland killing; “He caught lead poisoning. Heh, heh, heh.”
I’m surprised “6 ft under” hasn’t been mentioned yet!
Hand him a Bible (Torah, Koran) so he can study for the final?
Passed beyond the Rim (of course…)
Won’t be down for breakfast.
YO! Limey–that’s Mrs O’Leary’s cow, and she “caused” the Chicago Fire. Please make a note of it! 
RE “circling the drain”–that means not dead yet, but very soon will be around here.
“croak” I imagine comes from the weird breathing and the sigh that can occur immediately post-life.
here are some I’ve heard over the years in the hospital:
going upstairs
passing over
calling for Mama
bit the big one
was called home / was called over
went to that big [insert job title or occupation/place of residence here]
dust in the wind
finally at peace
at rest
flat lined
pronounced
SOL (shit out of luck) /TOD (time of death)
[QUOTE=eleanorigby]
YO! Limey–that’s Mrs O’Leary’s cow, and she “caused” the Chicago Fire. Please make a note of it!
Well at least I knew it was a cow
Carked it. Possibly derived from “carcass”. Strine, mate.
pushing up daisies
That’s part of NewZild as well. Goes for anything, man or metal, that’s bitten the dust.