Where to Begin on a Large Grid
Scan the borders and the corners: runs there cross fewer others, and their clues are often extreme. Two cells adding up to 17 are 8 and 9, three adding up to 6 are 1, 2 and 3. Every digit placed there shortens the runs that cross it, deeper in the grid.
Seven-Cell Runs
A seven-cell run uses seven of the nine digits, so its clue tells which two are missing: 28 means 8 and 9 are out, 42 means 1 and 2 are. Working with the missing digits, rather than the ones present, is the shortcut that makes long runs manageable.
Pencil Work on a Large Grid
A 10x10 kakuro rewards patience. A pencil and an eraser help, and so does writing the candidates of the hardest cells small in their corners; at Hard, expect to come back to the same crossing more than once before it gives way.