Loving-kindness is a meditation practice, which brings about positive attitudinal changes as it systematically develops the quality of 'loving-acceptance'. To put it into its context, Loving-kindness is the first of a series of meditations that produce four qualities of love: Friendliness (metta), Compassion (karuna), Appreciative Joy (mudita) and Equanimity (upekkha).
The practice always begins with developing a loving acceptance of yourself. If resistance is experienced then it indicates that feelings of unworthiness are present. No matter, this means there is work to be done, as the practice itself is designed to overcome any feelings of self-doubt or negativity. Then you are ready to systematically develop loving-kindness towards others.
Four Types of Persons to develop loving-kindness towards:
1) a respected, beloved person - such as a spiritual teacher; 2) a dearly beloved - which could be a close family member or friend; 3) a neutral person - somebody you know, but have no special feelings towards, e.g.: a person who serves you in a shop; 4) a hostile person - someone you are currently having difficulty with.
Starting with yourself, then systematically sending loving-kindness from person to person in the above order will have the effect of breaking down the barriers between the four types of people and yourself.
Ways of arousing feelings of loving-kindness:
1. Visualization - Bring up a mental picture. See yourself or the person the feeling is directed at smiling back at you or just being joyous; 2. By reflection - Reflect on the positive qualities of a person and the acts of kindness they have done. And to yourself, making an affirmation, a positive statement about yourself, using your own words; 3. Auditory - This is the simplest way but probably the most effective. Repeat an internalized mantra or phrase such as 'loving-kindness'
reference from: http://www.buddhanet.net/metta_in.htm