Pick one digit
Choose a number and scan every 3×3 box. Existing copies of that number eliminate rows and columns, often leaving a single valid cell in another box.
Repeat for digits one through nine. This cross-hatching method is especially effective at the start of an easy or medium puzzle.
Look for nearly complete units
A row with two gaps is more promising than one with seven. Compare the missing digits with the intersecting columns to place one or both.
Do the same for columns and boxes. Rotate between them instead of becoming stuck in one area.