You have one other command to use, if you really need it. You have a limited supply of Bombs.
If you get in serious trouble, press the Bomb key (initially set to the space bar) and a bomb will go off, forcing all the pistons to the top of the screen, allowing you more time to build a stack. Use bombs wisely, you don't have any to waste!
Scoring:
When you stop all the pistons in a level, you get the value of all the blocks remaining. So, don't just stop the pistons, but build tall stacks to get the best score!