Gathered from Gil Fronsdal's talk the other night, these simple instructions on what 'to do' while meditating - my notes from the talk, not verbatim:
Using the image of the cowherd looking after his cows, we are presented with two options when sitting, looking after our thoughts.
First, is if the cows are out while the rice fields are not yet harvested, the cowherd needs to keep track of his cows very closely so that the cows do not damage the crop. Similarly, there are times when we need to watch our mind and thoughts very closely. Telling ourselves, don't go there, don't do that. Being like a vigilant cowherd with his little stick.
The other way is taking on a relaxed attitude, being like the cowherd standing propped up against a tree, and simply keeping an eye on the cows roaming around. We are letting go of our thoughts as they come, not getting involved.
Part of the wisdom factor is to know when one way is useful and not the other. We learn this through trial and error.
In our modern Western culture, we are plagued with a variety of neuroses that may dictate which approach is generally best suited for our practice. If we are in the camp of 'I am inadequate, I can't do this', being told to practice diligently will be counterproductive and we will end up tying ourselves into a knot instead of becoming more free. What is needed instead in that case, is a message of radical acceptance. For others there is a risk of complacency if they let their mind think freely. This may result in supporting unhealthy attachments and perpetuating the state of bondage that keep us from finding inner freedom.
With proper attention, we notice the consequences of our thoughts and behaviors, and learn to discern which activities of the mind are skillful and which ones are not. To step back or to probe deeply and investigate like a surgeon, that's an individual matter.
Actually, not just an individual matter, to be settled once and for all, but rather a moment to moment decision that needs to be constantly re-evaluated depending on one's constantly changing states of mind. I know for myself, there are days when I tend to be more on the tense side, and a relaxed attitude is what I need to cultivate. Other times, when the mind is dull or not concentrated enough, I need to err more on the side of putting effort into concentrating and gathering my thoughts. Also, when the hindrances are strong and threaten to take over both mind and heart, I need to be extra vigilant and take a very active role with the thoughts. Hearing Blanche Hartman, "No, not getting into that train [of thoughts]."