One thing I've done in the past in a commercial game is simply to fill the screen randomly with 255,255,255 and 0,0,0 pixels. At 60Hz or better, you see shades of gray and the effect is convincing in short doses (I used it for a "changing the channels" kind of effect, with a bit of static between channels). Playing a white-noise sound at the same time is what really does it :)