Pause the game and press Select. This will make the preview window disappear when you resume play.
Turn the game on and let demo mode start. Press Select to bring up a swimming fish screen. Press A to return to demo mode, then press A again to bring up confetti and the Nintendo logo. Press B to return to demo mode, then press B again. You'll eventually see a chalkboard screen saver.
Fireworks Screen Saver:
Beat a high score.
Move blocks up:
While any piece except a straight bar or block is descending, hold Left or Right until it's against the wall. Now while still holding Left or Right, rapidly press A and the piece will start moving back up.
Special thanks to Revolution reader Bryan Lawrence!