I really hope this post grows and grows, because we can all use some fresh stuff! Here's a few:
* On O/H presses with plates, tell them that the plates are on a "first date" and no contact is allowed at the top.
* On chest presses, say they are like a nude beach...no top and no bottom. Also, during the chest track, I'll announce that I'm stamping out "man boobs" one chest press at a time! Or ladies, we are "lifting and separating!"
* Clean & Press...not just for laundry any more! Also right before a set of C&P, I'll ask a really stupid question like, "If you're happy and you know it, raise your hands" as the bar goes up or "If you want more, raise your hands, etc."
* Set position abs...hold them in like you're at your high school reunion.
* I'll tell one specific knock knock joke if I don't get much class participation: Knock, knock (who's there) Woo (woo who?) See? I told you I'd get you excited!!
* We've got our classes conditioned (or trained 
...) to respond to the question "Are your abs tight?" and they respond loudly "TIGHT!!" During crunches, on a 3/1, tell them they are laying on a beach and they really want to check out that cute girl/guy without their significant other catching them...so it's up, up, peek, then back down quick before you get caught!
* on push ups...I tell them to pretend there is an M&M on there mat right under their nose and they really, really want it!! Or, I'll ask them to smell the mat and the bottom and hope that the person that used it before them cleaned it really well!
* Squats...on Captain Jack, I'll sing over the chorus..."Na na na na na na, Baby squeeze your butt, squeeze your butt, squeeze it going up..."
Okay, probably more than anyone wanted, but I do have more!
The person that invented lunges was evil, evil, evil!!!