Effect: Defying all laws of leverage, you summon a ridiculous amount of strength.
Secret: This one is a win-win: The mark gets to experience an absolutely amazing effect while everyone else gets to laugh at the mark.
Start by facing each other. Have the mark hold two arms straight out in front of him. No bend at the elbows, arms parallel. Step forward and place his arms under your armpits, and ask him to try, as hard as he can, to lift you up off the ground without bending his elbows.
He won't even get close. The leverage is against him, and nobody has enough strength to overcome it. Make sure to give him ample opportunity to make it work... The more he realizes how impossible the task is, the more amazed he'll be when you pull it off.
Once he's convinced there's no way to lift you, switch positions: place your wrists under his armpits keeping your arms straight and parallel. Take deep breaths, put on a show of summoning all your strength and begin to push upwards. What he doesn't see is one of your friends sneaking up behind him, grabbing your fists, and pushing up on your arms lifting him several inches off the ground. To the mark it is simply as though you got freakishly strong instantly.
Summary: The reactions this effect gets are priceless. Some people will immediately want to lift you again, some people will believe you're that strong, and others won't know how to cope. Suggest they cope by buying you a beer.