Find a tall, empty styrofoam cup with at least a 14 oz. capacity. The taller the cup, the better the illusion will appear. If you plan to perform the trick at work, you can likely find the prop in your office.
Stick a very small piece of poster putty to the side of the cup. Be sure to work the poster putty between your fingers so that it will be soft enough to stick to your thumb, but not so sticky that you cannot remove it discretely. Place a thumb on the putty to make it stick to the cup.
Wrap your fingers around the cup and hold it casually. Slowly open your fingers and roll your thumb away from the cup slightly so it is not noticeable that your thumb is actually stuck to the cup. While doing this, push your hands slightly forward as if you are following the floating cup. Acting surprised that the cup is floating will add to the effect of your trick.
Wiggle your thumb and other fingers around the cup slowly. This will give the impression to the audience that the cup is not merely hovering but also dancing from hand to hand. Act as though the cup is trying to escape your grasp.
Conclude your act by "catching" the cup between your hands. Pour coffee or another beverage into the the cup, take a sip, and nonchalantly leave the room. Wink at your audience on your way out the door.