Cornered Pairs (Or, Look for the Big X.)

This technique works great if you spot a four sets of two numbers (In this puzzle, the numbers are 1 and 9.) that define the corners of a rectangle and can be connected by an X.

First, I'll use triangulation to find as many numbers as possible but will put in pairs of numbers as well. Let's start.

Since there is only a single "1," there is nothing I can do with it.

I can place a "2" in row 4 of column 1 and a ghost pair of "2s" in row 3, columns 4 and 6.

I can place ghost pairs of "3s" in rows 4 and 5 of column 2 and columns 8 and 9 of row 9.

I can place "4s" in column 4 of row 1 and column 8, row 4 and ghost "4s" in row 5, columns 2 and 3 and in column 4, rows 7 and 8.

You get the idea. Here are the results of one round of triangulation:

A second round of triangulation yields only "5s" in row 2, columns 2 and 3. The next step is to find row, columns and block with few blank spaces and hope to figure out what belongs. I'll start with the first block and first column: