This deceptively simple but powerful practice is one of my favorites. While walking, sitting, or reclining, count each exhalation of the breath. When you arrive at ten, start over. The beauty of this practice is that it has a built-in feedback monitor. If the mind wanders, you will keep counting past ten, or lose count entirely. When that happens, start over at one. I like to do this practice while walking, and often use it as a warmup for sitting. If, for example, I plan to do kasina practice (described below), I find it helpful to attain a concentrated state before sitting down. This saves me the usual five to ten minutes of fidgeting and allows me to get directly to work. How do I know I am concentrated enough? Because I was able to count to ten two or three consecutive times without losing count. It takes the guesswork out of concentration.
This is the gold standard practice for attaining "hard" concentration and jhana. A kasina is a colored disk that is used as a visual object. It doesn't much matter what color, but I favor pastels or earth tones. I use a cereal bowl. For years I carried around one of those cheap plastic bowls they use for bathing from tanks in Burma. Mine happened to be brown, about 8 inches in diameter. You prop the bowl up against the wall, sit a comfortable distance from it (about 5 to 8 feet) and stare at it. That's it. Your mind will go through the five chicken herding stages described above. At some point you will enter jhana. You will recognize it as an altered state of consciousness that feels very stable and very pleasant. Note that the first four jhanas correspond to chicken herding stages 2 through 5. Each jhana develops in the five stages, so it is like nested Russian dolls. Jhanas 5-8 are a subset of jhana 4, so there is always this nested relationship.
I have found both counting and kasina practices to be applicable to both retreats and daily practice at home. The more I go back and forth between deep concentration states and daily life activities, the easier it gets to make a quick and easy transition between them. In fact, there is a thing I sometimes do for my dharma friends that I call my "parlor trick," in which I sit down and cycle through all eight of the material and immaterial jhanas in less than two minutes. It doesn't look like much; I just sit there and shake and roll my eyes up into my head, holding up fingers to signal jhana numbers. (Although in the higher jhanas, I always forget which fingers to hold up and the signal system breaks down.) So they have to take my word for it that I attained all those jhanas. But I began doing it as a way to show people that jhanas aren't something abstract, or something for other people, but rather for ordinary people like us; they can be learned and cultivated to high levels and called up instantly, even during daily life. Also, I must admit, I began doing it as a way to rebel against a western Buddhist culture that teaches that it is wicked or shameful to admit that you "have the power of jhana." What rubbish.