Codes acquired from codes.ign.com
Tetris DX
Alternate screens
Press Select during demonstration mode to display an aquarium screen. Press A (x2) to 
return to demonstration mode then display a confetti screen. Press B (x2) to return 
to the demonstration mode then display a chalkboard. 
submitted by Billy G.

Ascending blocks
Hold Left until a piece touches the wall. Continue to hold d-pad Left and rapidly tap A. 
Alternately, hold Right and rapidly tap B. (Note: Cheat does not work on symmetrical 
pieces like the square and straight bar.)
submitted by Blazemore

Continue game
Press Start to pause game. Then turn Game Boy off. When the unit is turned on again, a 
Continue  screen will appear. 
submitted by Sushi Ball Z

Expert mode
Press Start during game. Then press Select. The preview window will not display the next 
piece when game resumes. 
submitted by phoenix9999

In-game reset
Press Start + Select + A + B during game. 
submitted by Joshmann