Pardon the terribly flawed words that follow. These are just some things I feel I have learned over the past few years of sitting that I felt were worth the effort to try and describe so that I could share it with you. It's no fun telling yourself this stuff.
At any one point during zazen when you realize you are thinking when you should be just open and aware you may judge your zazen to be not so good (“oops, I am doing it wrong”). And at some other point during zazen when you realize that you are just open and aware you may judge your zazen to be good (“oh, I am doing it right"). I have come to realize that both these judgments are mistaken because they are outcome thinking. Looking at any one point in time and making a judgment about it is outcome thinking. This is especially true in zazen. It closes the book, so to speak, and creates a thought as an endpoint judgment, an outcome.
But after your half hour is up and you look back on your zazen as a whole you may realize that thoughts came and went throughout, that zazen was/is/will always be a process. Even monkey mind and clear-sky mind (outcome judgments) come and go (process). Whenever I catch myself judging my zazen by whatever it is I am doing or non-doing in that moment I try and remind myself that this is outcome thinking and I am in a process here. This helps me drop it so that I can get on with the process.
Speaking of dropping things, we have all played the game of catch, right. Someone throws the ball, we follow the arc of the ball, and then we catch it. This might more properly be called a game of throw-catch, as that’s more descriptive of what is happening. I like to think of zazen as a game of catch-drop: as soon as I catch a thought I drop it. Also, interestingly enough, I have observed that this game is almost the direct inverse of the throw-catch game. Instead of seeing the arc and then catching it, I usually catch the thought and then see the arc of thoughts that led to me catching it. Of course when the process is going well I see the arc and the catching all at the same time. And when the process is going really well there is no arc, nothing to catch, and no one to catch all this nothingness. It all drops away.