How to actually get better, in order of what pays off first.
These are in the order that pays off. The first three will do more for your score than everything below them combined.
A flat stack accepts every blob. A jagged one only accepts a few, so you end up placing pieces where they fit rather than where they help, and the problem compounds. When you have a choice, take the placement that leaves the surface flatter, even if it scores nothing this turn.
A covered gap costs you every row above it: none of them can clear until you dig it out. One hole is survivable, three is usually the game. If you do create one, clear down to it early rather than building over it.
Leave one column open (traditionally the far right) and fill the other 9 to a good depth, then drop a Noodle for a Quad. It is worth 800 points against 400 for the same four rows cleared one at a time, and in versus it sends 4 rows instead of 0.
Two Quads in a row is not twice as good as one: the second is a streak, worth 1.5× points and an extra Sludge row. Once you can build one Quad reliably, practise resetting the Well so the next one is already half-built.
You do not need a repertoire. One reliable overhang you can build on demand turns a triple worth 500 into a Twist worth 1600, and triples what it sends (2 rows becomes 6).
When Sludge is pending, the clear that cancels it is usually better than the clear that is worth more. Look at the pending count beside your Well before you commit: matching it exactly means you take nothing and your opponent gains nothing.
Every row in a Sludge volley shares one gap column. Find it, and clear several rows with a single vertical blob rather than nibbling at the top. Digging fast is what buys you the time to start attacking again.
You can aim at a specific rival, counter whoever last hit you, hunt the tallest stack, or target the badge leader. Early on, pressuring the player who is already struggling ends them sooner and hands you their badges; late, the badge leader is the real threat.