The following is a Zen text that I find useful:
Rules For Meditation
Why are training and enlightenment differentiated since the Truth is universal? Why study the means of attaining it since the supreme teaching is free? Since Truth is seen to be clearly apart from that which is unclean, why cling to a means of cleansing it? Since Truth is not seperate from training, training is unnecessary- the seperation will be as that between heaven and earth if even the slightest gap exists FOR WHEN THE OPPOSITES ARISE THE BUDDHA MIND IS LOST.
However much you may be proud of your understanding, however much you may be enlightened, whatever your attainment of wisdom and supernatural power, your finding of the way to illumination, your power to touch heaven and to enter into enlightenment, when the opposites arise you have almost lost the way to salvation. Although the Buddha had great wisdom at birth, He sat in training for six years; although Bodhidharma Transmitted the Buddha Mind, we still hear the echoes of his nine years facing a wall.
The ancestors were very dilligent and there is no reason why we people of the present day cannot understand. All you have to do is cease from erudition, withdraw within and reflect upon yourself. Should you be able to cast off body and mind naturally, the Buddha Mind will immediately manifest itself; if you want to find it quickly, you must start at once.
You should meditate in a quiet room, eat and drink moderately, cut all ties, give up everything, think of neither good nor evil, consider neither right nor wrong. Control mind function, will, consciousness, memory, perception, and understanding; you must not strive thus to become Buddha. Cling to neither sitting nor lying down.
When meditating, do not wear tight clothing. Rest the left hand in the palm of the right hand with the thumbs touching lightly; sit upright, leaning neither to left nor right, backwards nor forwards. The ears must be in line with the shoulders and the nose in line with the navel; the tongue must be held lightly against the back of the top teeth with the lips and teeth closed. Keep the eyes open, breathe in quickly, settle the body comfortably and breathe out sharply. Sway the body left and right then sit steadily, neither trying to think nor trying not to think; just sitting, with no deliberate thought, is the important aspect of Serene Reflection Meditation, (Zazen).
This type of meditation is not something that is done in stages; it is simply the lawful gateway to carefree peace. To train and enlighten ourselves is to become thoroughly wise; the koan appears naturally in daily life. If you become thus utterly free you will be as the water wherein the dragon dwells or as the mountain whereon the tiger roams. Understand clearly that the Truth appears naturally and then your mind will be free fromm doubts and vacillation. When you wish to arise from meditation, sway the body gently from side to side and arise quietly; the body must make no violent movement.
I myself have seen that the ability to die whilst sitting or standing which transcends both peasant and sage, is obtained through the power of Seren Reflection Meditation. It is no more possible to understand natural activity with a judgemental mind than it is possible to understand the signs of enlightenment; nor is it possible to understand training and enlightenment by supernatural means; such understanding is outside the realm of speech and vision, such Truth is beyond personal opinions.
Do not discuss the wise and the ignorant, there is only one thing- to train hard for this is true enlightenment; training and enlightenment are naturally undefiled; to live in this way is the same as to live an ordinary daily life. The Buddha Seal has been preserved by both the Buddha's in the present world and by those in the world of the Indian and Chinese Ancestors, they are thus always spreading the Truth- all activity is permeated with pure meditation- the means of training are thousand fold but pure meditation must be done.
It is futile to travel to other dusty countries thus forsaking your own seat; if your first step is false, you will immediately stumble. Already you are in possession of the vital attributes of a humam being - do not waste time with this and that, you can possess the authority of Buddha.
Of what use is it to merely enjoy this fleeting world? This body is as transient as dew on the grass, life passes as swiftly as a flash of lightening, quickly the body passes away, in a moment life is gone. O sincere trainees, do not doubt the true dragon, do not spend so much time in rubbing only a part of the elephant; look inwards and advance directly along the road that leads to the Mind, respect those who have reached the goal of goallessness, become one with the wisdom of the Buddha, Transmit the wisdom of the ancestors. If you do these things for some time you will become as herein described and then the Treasure House will open naturally and you will enjoy it fully.
Great Master Dogen (12th century)