In addition to basic Tetris play, what strategies are critical to winning Tetris DS in tournament play (items on, best 2 out of three)?
In no particular order:
Number 1 is important because if you can T-Spin fast, you can completely ignore Tetrises for garbage-sending. It also helps for #3 when you need to clean up garbage; it's an easy way to get something back.
Two is important because a T-Spin Triple or Tetris back-to-back will send you up six or so rows, which can easily screw you over if you're already high up.
Other than that, just be fast and don't lose your cool.