Take time to observe
In brief
After each placement, check the row, column, and both diagonals. Defending one line can leave a threat on another.
Look in all four directions
After each placement, check the row, column, and both diagonals. Defending one line can leave a threat on another.
Spot open fours
Four consecutive stones with both ends open create two winning moves. Prevent this shape before you have to defend both ends.
Weigh attack against urgency
Before preparing your own sequence, check whether your opponent can already win on the next move. A single point may take priority over every other plan.