Depends how hard you're willing to go with things, but if you get a packet of sherbet, a fizzy soda and pale your skin a little bit or make yourself look clammy (try holding an ice cube in your hands for a while, and rubbing it over your face/arms) then put the sherbet in your mouth, take a sip of soda (not too much, it's quite a violent reaction) and then start shaking your limbs and shoulders, and allowing a little bit of the froth created in the reaction to dribble down your mouth... Fall to the floor, and try not to focus on anything near to you (desks, chairs, people) instead focus on a corner of a wall, occaisionally dipping your head and losing sight of the object for a second.
Downsides to this are: Shots of adrenaline, A&E room trips, or being found out.
