Back-to-Back Chaining Methods

Tetris
BtB tetris is probably the most widely used tactic of them all. A player simply builds up for one tetris. After clearing with an I, a player stacks on top of the foundation for a second tetris.

T-Spin Single
T-spin singles are the easiest to spot and to exploit in multiplayer. Not utilizing them will only put you at a disadvantage against an opponent who does. In Tetris Worlds, T-spin singles with the T tetromino's flat side against the wall don't send garbage, but in Tetris DS, they do.

If a situation is not readily available, create one if you don't already have a BtB chain going.

These work even on Tetris Worlds because the T's back is against other blocks, not the wall:

T-Spin Double
T-Spin Doubles, when combined with tetrises, produce one of the most deadly combinations in multiplayer.

The article ST stacking describes a stacking method that uses several consecutive T-spin doubles. Though designed for single player, it can also prove useful as an opening in multiplayer.