破釜沉舟 or "break the cauldrons and sink the boats"; to commit oneself irrevocably
During the late years of the Qin Dynasty (221-206BC), Xiang Yu led a rebellion. After crossing the Zhang River, Xiang Yu ordered his men to sink all their boats and break their cooking pots. He issued each soldier three days' rations and warned them that there was no way to retreat; the only thing they could do to survive was to advance and fight. After nine fierce battles, the Qin army was finally defeated.
The idiom is used to indicate one's firm determination to achieve one's goal at any cost.
Cortez also burned his boats for the same reason. Hopefully someone can think of a comparable idiom that isn't based on an impending war crime.
One of my favorite sayings at work. Or a variation on it: the pooch has been thoroughly screwed (usually followed by, “I’ll go page for maintenance.”)
I like "someone's walking on my grave." because of the implied supernatural chain of events, and because it's so fuckin' extra.
It means basically "I've got a feeling something bad will happen." Someone's walking where I'll be when I'll die, which gives me the supernatural ability to sense impending misfortune. It implies an almost delightfully mystical worldview that collapses the separation between life and death and the present and the future. Then there's the it's an anglo saying, but the yankee version "a goose is walking over my grave." which ads an extra level of folksy.
Oh, and you say it when you get a sudden shiver that lets you know something bad will happen specifically. It's the extra level of specificity that really does it for me.
I love the original: don't teach your grandmother to suck eggs. It sounds sexual, but isn't, but works either way. The ambiguity is the charm.
Goldfish poop (Japanese phrase referring to annoying hanger-on types, cracks me up no matter how many times I see it)
Chinese history idioms are always pretty interesting too.
this one is stupid and corny and straight from the south, but it falls out of my mouth way too often. "almost-chicken don't make gravy." jokes on you dad, it does
"Picking gnat shit out of pepper". To be overly concerned with minor or inconsequential details.