For some this will be an obvious observation, but, I’ve won more than a couple games because the other player has overlooked or forgotten this important point. And it is this:

Every move a person makes, after the very first one, involves placing two (2) stones; therefore, if you finish your turn and your opponent still has a line of four (4) stones with 2 open spots [on one side or the other or in combination] they will be able to win on their next turn.

And creating a situation where your opponent cannot block all of your four stone attacks on their next move is how to ensure your win, beyond hoping your opponent misses one of your attacks.

P.S. When blocking a four stone attack, sometimes it’s desirable to put one of your blocking stones one space off [letting your opponent stretch the line to five stones but no further] because you can use that blocking stone in a future attack of your own.