Meditation

Many martial artists are meditating and considering the meditation to be a very important part of their training. And likewise: many meditation teachers are encouraging their students - at least the more advanced practitioners - to exercise regularly, using training systems strengthening the energy circuit (i.e., Tai Chi, Qi Gong, Yoga etc). You can say that disciplines such as martial arts and meditation have a certain interconnectedness.

There are many meditation techniques, and they all have some positive effects, e.g. relaxation, a better power of concentration and stress relief.

Which method to choose as a martial artist is a matter of what positive effects the techniques have, and which of the positive effects is considered the most beneficial to a martial artist.
The methods described below are the ones I have been recommended as the most beneficial to a martial artist. I'm also supplying the background knowledge I consider to be necessary - if you prefer an even deeper knowledge, please refer to other resources.
I won't be giving detailed instructions on how to meditate (if that is required please refer to other resources or a meditation teacher). But I will point out that the breathing have to be deep and calm, going all the way down into the belly. And that you have to be very concentrated and focussed, but at the same time relaxed, i.e. the effort has to be just as relaxed as the rest of the body (and the breathing); this is one of the paradoxes of meditation, and something we have to learn to live with - there are no easy shortcuts, and we can't force progress to happen, we just have to take is easy and wait for the improvements to happen over time.

The one of the "core" meditation techniques that is considered to be the most beneficial to a martial artist is the method known as mindfulness meditation - the method where the main focus is the breathing.
This method is very good for increasing and expanding awareness as well as for developing a clear focus and being totally present - which also happens to be the positive effects of meditation most beneficial to a martial artist.
This method is said to be the method taught by Gautama Buddha. Mindfulness is the practice whereby a person is intentionally aware of his or her thoughts and actions in the present moment, non-judgmentally. Actually the correct term is Anapanasati (Pali), a term that is composed of two words: sati that can be translated into "mindfulness"; and anapana that refers to inhalation and exhalation (i.e. the breathing). Simplified the term can be translated into "mindfulness of breathing". Of course Anapanasati isn't the only type of mindfulness meditation, there are other methods with another main focus than the breathing - but since this method is the most common it is usually the method referred to when using the term "mindfulness meditation".

Another method that is very beneficial to a martial artist is focussing the attention on hara ("belly" in Japanese), located in the lower part of the abdomen two finger-widths below the navel and approximately one third into the abdominal cave. This point is considered to be the physical centre of gravity and a vital energy point, that is very important to the energy circuit. I'm using the Japanese term since it is the most well-known and used term. This point is also referred to using other terms, e.g. tai-tien, dantian, qihai (all Chinese) and chi point, in addition the point is equivalent to the 2. chakra ("wheel" in Sanskrit) or navel chakra from the East, e.g. used in Yoga and Tantra meditation.
Dantian (also referred to using other terms, e.g. tanden in Japanese) can be translated into "cinnabar field" or "elixir field". This term is typically referring to the point also known as hara; by focussing the attention on this point it is possible to bring back chi and keep chi circulating. But dantian is also a more fundamental term describing the chi circuit, that can be divided into more vital points - typically dantian is divided into three primary dantian: the lower (equivalent to hara) associated with physical energy and sometimes also sexuality; the middle (located at the heart area and equivalent to the 4. chakra) with significance to emotions and the health of the internal organs; and the upper (located between the eyebrows, i.e. "the third eye" and equivalent to the 6. chakra) with significance to the brain, cognitive thinking and consciousness.
It isn't necessary to do a complete dantian meditation alternating between all the points (even though that is of course a possibility). But focussing the attention on the lower dantian (or hara), a point that is (as mentioned above) considered the centre of gravity and a vital energy point, will be very rewarding to a martial artist. It isn't easy to focus the attention on this point since it is something we have to feel naturally (most practitioners won't be able to do that in the beginning), in the beginning it might be helpful to focus on the abdominal wall moving out and in when breathing - this awareness might make it easier to establish a focus point.

Dantian meditation is my preferred method since it is effective and it could also be beneficial to runners - but I have to emphasize that it is a more advanced method than e.g. mindfulness meditation. In addition to the methods described above I'm also practicing other meditation techniques that isn't related to the martial arts training.

It doesn't have to be time consuming to meditate. 10-15 minutes of meditation a day is sufficient to make a difference. It is well documented that as little as 20 minutes of meditation 5 days a week will result in a noticeable stress relief.