Meditation can be considered a technique, or practice. It involves concentrating on God.Over time, the number of random thoughts occuring diminishes. More importantly, your attachment to these thoughts, and your identification with them, progressively become less. He who practices meditation without ethical perfection, without the practice of Yama-Niyama cannot obtain the fruits of meditation. Purify your mind first through the practice of Yama-Niyama. Then practice regular meditation. Then you will attain illumination. Meditation on OM with Bhava and its meaning removes obstacles in Sadhana and helps to attain Samadhi. "Raja Yoga is a practical guide for gaining control over the mind. The second sutra (of Patanjali's Yoga Sutras- Ed.) states, "Yogas chitta vritti nirodhah," or,"The restraint of the modifications of the mind-stuff is Yoga." Every thought, feeling, perception, or memory you may have causes a modification, or ripple, in the mind. It distorts and colors the mental mirror. If you can restrain the mind from forming into modifications, there will be no distortion, and you will experience your true Self." There are various types of meditation - prayer is probably the best known, but there is also TM (Transcendental Meditation), mindfulness meditation, and from the Eastern tradition, Zen meditation, Buddhist meditation, and Taoist meditation. 

Retention of breath(Pranayama), Brahmacharya, Sattvic food, seclusion, silence, Satsanga, not mixing much with people are all aids to concentration on god or Meditation.Never miss a day in meditation. Regularity is of paramount importance. When the mind is tired, do not concentrate. Do not take heavy food at night if you wish to meditate.