Mindfulness has been shown to be very effective and useful in reducing stress, anxiety, worry and anger - amongst other things - and to be useful in treating depression. It is often talked about as a "technique" although it is much more than that. While it can be useful as a technique to manage feelings and reactions in the moment, properly learned and practiced, it becomes a long-term skill that can transform patterns of reactions (e.g. those related to stress, depression, anxiety, worry and anger) and reduce the frequency and intensity of these reactions, but actually changing your brain.
The essence of mindfulness is a practice of holding your attention on something that is occurring in the present moment, and by doing so, holding your attention on an awareness of your experience in the present moment, while staying out of thinking, and of reacting.
Much of our mental activity is taken up with thoughts about the past or the future, thoughts that go over things that have already happened, or things that have not yet happened, thoughts that bring us up, or bring us down. These are the thoughts that contribute to stress, anxiety, depression, etc. They are rarely very useful, especially when the thinking process is not purposeful and intentional. When our thoughts are just "churning" on their own, especially over ground that has already been covered, this is usually not productive and it can be the cause of stress and tension. It usually interferes with, rather than facilitates, solving any problems at hand, and it distracts us from our experience of the present moment, which is the only moment in which we are actually existing, and in which we can act.
The practice of mindfulness usually starts with simply sitting and focusing attention on the breath. This brings your focus away from your thoughts and thinking, and gives you something immediate, physically felt and experienced, to focus on. You are simply tuning in the physical sensation of breathing each breath, and every time that you find that your focus, your attention, is distracted by thoughts – or anything else – you simply bring your focus back to the breathing. Mindfulness practice can extend to many other aspects of everyday life, but this is a good starting point.
The more you practice this, the more that you are cultivating your ability to voluntary switch your focus from unproductive thinking, to your experience of being in the present moment. You are not only creating a habit through conditioning; you are actually rewiring your brain in the process. At the same time, holding your attention on your breath increases your awareness of what you are feeling internally – not just your emotions, but physical tension and overall well-being. Practiced consistently over an extended period of time, mindfulness becomes an incredibly powerful method of attending to your mental and physical well-being, managing your thoughts and feelings, being available to live your life in the present moment, and access the resources that you have.
I have been practicing mindfulness in a variety of ways, personally and professionally, for over 20 years, and often incorporate it in working with clients, when useful and appropriate.