Every time your opponent moves, you should do some serious thinking.

If he played the move you expected him to play,

You can probably continue with your plan.

If, however, he did something else, many other questions need to be asked:

That does that move accomplish for him?

Which squares does that piece attack?

Did it uncover any new lines of attack?

If this move forces a reply, see if you can devise a

defensive maneuver that also has attacking prospects.

Pay close attention to anything the other player may have up his sleeve,

even more so than to your own well-laid plans.