Okay...here's a couple. In squats, I was trying to come up with a visualization for squeezing the cheeks on the way up. Try this: tell your class there is a pile of money on the floor, and they are trying to pick it up with their "cheeks". They don't want to drop it on the way up, so squeeze tight!!
For chest, I tell people that we are working fixing "bra overhang" (women know what that is...) and men, we're working on "man boobs". Then I tell them I hope no one has a problem with both! :)
The person that invented lunges was evil, evil, evil!!!