If you missed the first two parts of this series, they are here and here.
As I said in the last post, you could continue just following the breath and find enough material for a lifetime of practice. But in case that's not quite enough for you, here are some things we do in my school (that's the Kwan Um School of Zen, if you're interested) to focus the attention further.
I like to use the breath initially as a centering device anytime I sit, regardless of the technique I intend to use afterwards. This centering can take anywhere from a minute or two to a half-hour, depending on my mental state at the time. After that though, I often use a phrase, a mantra, as a sort of object of meditation. This technique is widely taught in our school, and many people find it useful.
The process of self-inquiry is central to Buddhism. Each of us carries a set of beliefs about ourselves, a roster of who and what we are. I like to compare it to a big bucket, lik eone of those orange plastic things from Home Depot. In the bucket we carry around all the things we think of as "I."
Each or us has a name and a body, and these are closely associated in our minds with who we are. In reality however, our name is just our body's name. We could just as easily have a different name and still be the same person. As Shakespeare tells us in "Romeo and Juliet," A rose by any other name would smell as sweet. So Romeo would, were he not Romeo called, retain that dear perfection which he owes without that title. I could change my name, couldn't I? Who would I be then?
The message here is, I have a name, but I am not my name.
Even our body is not really who we are. Maybe you enjoy weight lifting. But you are not a different person when your muscles are well-defined than you were when you were out of shape. (It's like the old saying, "Sports do not build character; they reveal it." Fit or fat, it's still you! ) Professor Stephen Hawking is a wonderful example of someone who has a body, but is not defined by that body. To be sure, most people initially think of him as "that guy in the wheelchair." But the brilliance of his scientific work has come about despite his physical infirmities, not because of them. Reading his books, it is easy to forget that he is also "the guy in the wheelchair." Professor Hawking has a body, but he is not his body.
So who am I?
In my own case, I am a husband, a father, a son, a brother, a friend, a student, and a teacher (to name just a few things). These relationships take up a large part of my attention, time and energy. Perhaps they are appropriate answers to the question, "Who am I?"
Relationships change all the time. People die. Babies are born. We lose friends and make new ones. Couples divorce. As my relationships change, must I then alter my answer to the question, "Who am I?" Am I defined by my relationships?
I am also a writer, an actor, a videographer, and a few other things. Maybe you are a stock broker, or a carpenter, or a truck driver. But these are all things we do, not what we are. When we retire from our professions, do we cease to exist? Sadly, for too many people, the answer seems to be yes. Too many of us invest so much in our jobs that when we stop working, we just sit around waiting to die. But this is really just a failure to explore the question "What am I?" all the way to its end.
When you unpack "What am I?" all the way to the bottom of the orange bucket, you find the bucket is completely empty. You don't even have "nothing" in the bucket, because "nothing" is a thing too. In Zen, when we get to the bottom of the bucket and see what's left, we call it, "Don't know." Not "I don't know," because who is this "I" you're referring to? But just "Don't know." And all of Zen practice is based on getting comfortable with "Don't know."
So we use "Don't know" as an object of meditation. Here is one way to do this: on the in-breath, rest your attention on the phrase, "What am I?" Some people--deep breathers, I guess--will actually repeat the phrase several times silently as they breathe in: "What am I? What am I? What am I?" Then on the out-breath, we say, "Don't kno-o-o-o-o-w."
Try using this as the object of your meditation. Envision the different components of your personal image of who you are--your body, your relationships, your job, whatever is appropriate in your case--unpack all of these things and see whether they really are you. I predict that eventually you will get to the bottom of your bucket. When you reach that stage, you can begin to get comfortable with "Don't know."
If you have questions about this practice (or any other), feel free to ask them here. I don't promise to have all the answers, but I'll do my best.