Solve hard Sudoku methodically
Hard puzzles require comparing candidate positions across units. Locked candidates are especially useful: if a digit inside a box can only occur in one row, it can be removed from the rest of that row outside the box.
Naked pairs can remove candidates from other cells. A useful deduction may eliminate an option rather than place a digit immediately.
Make progress without guessing
After each elimination, look for naked and hidden singles again. Combining a small set of proven techniques often unlocks a stuck grid.