Flat stacking and the open column
Three habits do most of the work in Dockside Crates: a flat surface, one open column and a sensible use of Hold.
Why flat wins
Look at the top edge of your stack as a skyline. Steps of one square accept almost every crate; a step of two takes only a few shapes; a step of three or more takes only the bar. Every time you place a crate, prefer the spot that leaves the skyline smoother, even if it does not clear a row right away.
Holes — empty squares with crates above them — are the other enemy. A row with a hole cannot clear until you dig down to it, so do not cover one by choice.
The open column
Keep one column at the side empty and stack the other nine flat. When the stack is four rows high, the long bar drops into the column and clears four rows at once — 700 points times the level, compared with 400 for four single rows.
Crates come in sets of seven with every shape once, so a bar is never far away. In the worst case you wait twelve crates; usually far fewer.
Using Hold
Hold swaps the falling crate with the one set aside. Use it to keep a bar ready for the open column, or to park a shape that fits nowhere right now. Hold works once per crate, so do not rely on swapping back and forth.
The next three crates are shown beside the dock. Plan your current crate so that the following one has a good place too.
Speed
As the level rises, the time to think shrinks. Make your decision while the crate is still in the preview, then move and drop it at once. In Sprint 40 a hard drop as soon as the shadow is in place is worth more than a perfect placement found too late.