Candidates in Small Cells
Keep a sharp pencil for candidates: on a smaller grid, writing two or three possible digits in a corner of a cell keeps the crossings readable.
Practice That Builds Up
Solving four grids of the same level in a row trains the eye: the extreme sums, like 16 in two cells or 23 in three, start to jump out of the page, and so do the crossings where a single digit fits both runs.
From Four Small Grids to One Large One
When a Hard sheet of four grids feels comfortable, the next step is the single 10x10 grid, which uses the same reasoning with longer runs. The 8x8 Kakuro page prints this size one grid at a time.