The easiest way to play is to build stacked blocks against either the left side or right side of the screen while using a trash pile (aka red-green builder, with a yellow piece at the base) on the opposite side. Here is a screenshot that illustrates this technique:
A more in-depth discussion by Aaron Pesnell (including the concept of peaks and valleys in the trash) can be found at the Tetris Lovers homepage. It is our opinion that you won't be able to do much better than 51 seconds with this method (although it yields good Sprint results). Nevertheless, it constitutes a nice approach for newcomers to the game, allowing them to focus on dropping pieces quickly.
The theoretical best time for Ultra is 25 seconds: assume you get a perfect set of pieces consisting of 20 blue sticks. It will take 20 seconds to drop those 20 pieces, a total of 4 seconds to fuse the sticks into gold blocks (1 second per block) and 1 second to clear the bottom 2 blocks with a tetris (note: the last tetris used to clear the upper 2 blocks takes no time, since the game immediately ends). This will yield 170 lines.
The above makes it clear that to get a good time, instead of building trash on one side and blocks on the other, you should build blocks on both sides of the screen.
To obtain the required 150 lines in Ultra, the following combinations of blocks and clearing methods are sufficient:
With your opening set, you should generally build two partial blocks one above the other on one side of the screen. The reason for this is so that you have a platform above the second block on which to place trash. Once you get two partial blocks on the other side, you can place trash there as well. When creating your trash heaps, build vertically first. Building vertically saves time since it takes less time to drop a piece from the top to a platform near the top.
The maximum number of lines you can get is 149 + 85 = 234. The last 85 is 2 golds side by side with a tetris by a blue stick.