Fiddlesworth

This stacking method, discovered by fiddlesworth, produces 12 garbage lines in Tetris DS and Tetris Online (Japan), while using roughly half as many tetrominoes (15 to 21) as the common "three tetrises" opening (30 to 35). It requires Hold piece, SRS, 3-corner T-spin recognition, and 7-piece bag randomizer.

You can perform this stack from an empty playfield if any of the following is true of the first bag of seven tetrominoes: It is especially useful if your first tetromino is S or Z, but it does have a speed penalty on games with a slow soft drop (like Tetris DS).
 * S before J
 * Z before L
 * J before S before T (hold the J)
 * L before Z before T (hold the L)

T-spin Single
Start with an S–J or Z-L:

Build it (S and J depicted)

Soft-drop a T and spin it:

T-spin Triple
Build it more. You'll need to soft-drop the S, and you'll need to switch the O and Z if you got Z before O in the first bag:

Soft-drop a T and spin it twice:

T-spin Single
Make another T-spin single. If your stack doesn't look exactly like this, don't worry. You're pretty much improvising at this point. This will send an additional 3 lines of garbage, bringing the total to 12 lines sent.

Adjusting Your Strategy
Sometimes the 2nd T-spin single needs to be skipped. If you've received a lot of garbage, you may need to avoid overstacking and find the quickest path to uncover the garbage and clear it away. Other times, the garbage you've received may make the 2nd TSS impossible (or may open up better possibilities):

Open Corner
This is not a 3-corner T-spin, so no reward is given.

There's a better way to handle this situation:

Lucky Hole
Here you can make a back-to-back T-spin double after the T-spin triple with a little help from the garbage randomizer:

Continuations
There are several ways to continue making T-spins following single-triple-single.

T-spin Double
 Type A 

 Type B 

 Type C 

T-spin Triple
 Type A 

 Type B 

Yoshihiro
 Type A 

A1:

A2: