It really all depends on what you mean by sleeper. If you just don't want the extra attention of those who are clueless about cars just do the whole de-badge, de-wing, de-scoop method.
Now if you mean you want to street race, but you want to try and fool others who might not be so savy... Do all of the same as above, but then add the most obnoxious wing, the cheapest vinyl graphics, some Type-R badges, and some neon lights (inside and outside) with the bulbs showing. Walk around talking about if you had NOS you would be the fast MFer in the Tri-State area. You might have some people anxious to race then. Ricers are always a fun target for racers; you just gotta rice it all the way out or they might pay better attention. Oh, and don't forget to have a cheap stereo wtih a 15" sub that does nothing but rattle the car.