In general, any virtuous object can be used as an object of meditation.
If we discover that by acquainting our mind with a particular object our mind becomes more peaceful and virtuous, this indicates that for us that object is virtuous. If the opposite happens, for us it is a non-virtuous object. Many objects are neutral and have no particular positive or negative effect on our mind.
There are many different virtuous objects of meditation, but the most meaningful the objects of the twenty-one meditations, from meditation on relying upon a Spiritual Guide to meditation on emptiness, the ultimate nature of phenomena. Explanations of these can be found in the The New Meditation Handbook.
By relying upon a qualified Spiritual Guide we open the door to practicing Dharma. Through the blessings of our Spiritual Guide we generate faith and confidence in our practice, and easily attain all the realizations of the stages of the path. For these reasons we need to meditate on relying upon a Spiritual Guide.
We need to meditate on our precious human life to realize that we now have a special opportunity to practice Dharma. If we appreciate the great potential of this life we shall not waste it by engaging in meaningless activities.
We need to meditate on death and impermanence to overcome procrastination, and to ensure that our Dharma practice is pure by overcoming our preoccupation with worldly concerns. If we practice Dharma purely it is not very difficult to attain realizations.
By meditating on the danger of lower rebirth, taking refuge sincerely, and avoiding non-virtue and practicing virtue, we protect ourself from taking lower rebirth and ensure that life after life we shall obtain a precious human rebirth endowed with all the conditions conducive to the practice of Dharma.
We need to meditate on the sufferings of humans and gods so that we develop a spontaneous wish to attain permanent liberation, or nirvana. This wish, known as ‘renunciation’, strongly encourages us to complete the practice of the spiritual paths, which are the actual methods for attaining full liberation.
We need to meditate on love, compassion, and bodhichitta so that we can overcome our self-cherishing and develop and maintain a good heart towards all living beings.
With this good heart we need to meditate on tranquil abiding and superior seeing so that we can eradicate our ignorance and finally become a Buddha by abandoning the two types of obstruction.