Higher Levels
Hold A as you select your level, and you'll start 10 levels ahead of the one you selected.

Finish level
In the B-type of the game, where you have to remove 25 lines to end the level, wait until your block hits the ground. Then press the buttons 'A' and 'select' at the same time; this will finish the level.

In the A-type of the game, doing this will grant you 10.000 points, while when you're at 90.000 points, you'll see a bug, namely: A0.000 instead of 100.000 points.