"It's raining like cow piss on a flat rock." It's raining very hard.
"Slicker than greased goose $hit on glass." Usually refers to icy roads, but could be anything slick or slippery.
"I'll mop the floor with you." A threat, not an idle one either.
"Mercy stars!" An expletive, on par with "Oh my goodness!"
"Sam hell;" as in: "What in the Sam Hell were you thinking!?!" I always wondered who this Sam was, but I knew better than to ask.
"Quicker than greased lightning." Very fast or nimble.
"Hide & watch." Wait and see.
"Tight as a frog's ass." Refers to anything tight.
"Clear as mud." Usually refers to complex instructions.
"Use it up, wear it out, make it do or do without." Pretty much self explanitory, but it means you don't throw anything away if it can still be used for something. For example: The clock on your clock radio is broken, but the radio still works; don't throw it away, just use it as a radio & get a new clock.