The Practice of Lovingkindness (Metta)

The following is from the accompanying Meditations for The Buddha is Still Teaching: Contemporary Buddhist Wisdom Edited by Jack Kornfield
The Revolutionary Art of Happiness
In doing metta practice, we gently repeat phrases that are meaningful in terms of what we wish, first for ourselves, and then for others. We begin by befriending ourselves. The aspirations we articulate should be deeply felt and somewhat enduring (not something like “May I find a good show on television tonight”). Classically there are four phrases used:
May I be free from danger.
May I have mental happiness.
May I have physical happiness.
May I have ease of well-being.
You can experiment with them, alter them, or simply choose an alternative set of three or four phrases. Discover personally in your own heartfelt investigation what is truly significant for you.
May I be free from danger.
We begin to extend care and lovingkindness toward ourselves with the wish that we may find freedom from danger, that we may know safety. Other possible phrases are “May I have safety,” and “May I be free from fear.”
May I have mental happiness.
If we were in touch with our own loveliness, if we felt less fearful of others, if we trusted our ability to love, we would have mental happiness.
May I have physical happiness.
You might also use a phrase such as “May I be healthy,” “May I be healed,” “May I make a friend of my body,” or “May I embody my love and understanding.”
May I have ease of well-being.
This phrase points to the exigencies of everyday life—

concerns such as relationships, family issues, and livelihood. With the expression of this phrase, we wish that these elements of our day-to-day lives be free from struggle, that they be accomplished gracefully and easily. Alternative phrases could be “May I live with ease,” or “May lovingkindness manifest throughout my life,” or “May I dwell in peace.”
Sit comfortably. You can begin with five minutes of reflection on the good within you or your wish to be happy. Then choose three or four phrases that express what you most deeply wish for yourself, and repeat them over and over again. You can coordinate the phrases with the breath, if you wish, or simply have your mind rest in the phrases without a physical anchor. Feel free to experiment, and be creative. Without trying to force or demand a loving feeling, see if there are circumstances you can imagine yourself in where you can more readily experience friendship with yourself. Is it seeing yourself as a young child? One friend imagined himself sitting surrounded by all the most loving people he had ever heard of in the world, receiving their kindness and good wishes. For the first time, love for himself seemed to enter his heart. Develop a gentle pacing with the phrases; there is no need to rush through them or say them harshly. You are offering yourself a gift with each phrase. If your attention wanders, or if difficult feelings or memories arise, try to let go of them in the spirit of kindness and begin again repeating the metta phrases:
May I be free from danger.
May I have mental happiness.
May I have physical happiness.
May I have ease of well-being.
There are times when feelings of unworthiness come up strongly, and you clearly see the conditions that limit your love for yourself. Breathe gently, accept that these feelings have arisen, remember the beauty of your wish to be happy, and return to the metta phrases. 
There is no need to worry about what has gone by or to anticipate what has not yet come, not even the next phrase. Don’t struggle to manufacture a feeling of love. Simply repeat the phrases, thereby planting the potent seeds of intention, and trust that nature will take its own course.
Next extend your lovingkindness to a benefactor, someone for whom you feel gratitude and respect. It is best to start out using the same phrases you have directed toward yourself, to begin to break down the barriers between self and other: “Just as I want to be happy, so do you want to be happy. May you be happy.” If over time the phrases modulate to fit the particular being, that is fine.
We say the phrases as though cherishing a fragile, precious object in our hand. Were we to grab on to it too tightly, it would shatter and break. Were we to be lax and negligent, it would fall out of our hand and break. We cherish the object gently, carefully, without force but paying close attention. Try to connect to each phrase, one at a time.
Now use the same phrases you have used for a neutral person. Choose someone whom you tend to see occasionally, since that will bring them and your changing feelings for them into clearer focus. He or she is a generic living being, wanting to be happy just as all of us do, making mistakes just as all of us do. Reflect on the neutral person’s wish to be happy, identical to your own, and direct the metta phrases toward them.
Over time, it is common to discover an increase of caring and warmth toward the neutral person, as they seem closer and closer to you. They are, after all, a kind of nonerotic secret love.
Now send your metta to a person with whom you experience conflict, fear, or anger—known in the traditional texts as the enemy. You can reflect on this line from Rainer Maria Rilke: “Perhaps everything terrible is in its deepest being something that needs our love.”
It is useful to begin with someone with whom the difficulty is relatively mild—not starting right away with an attempt to send metta to the one person who has hurt us the most in this lifetime. It is important to approach increasingly difficult people gradually.
Perhaps you can most easily feel metta for a difficult person if you imagine them as a vulnerable infant, or on their deathbed (but not with eager anticipation—be careful). You should allow yourself to be creative, daring, even humorous, in imagining situations where you can more readily feel kindness toward a difficult person.
Gently continue to direct metta toward the difficult person, and accept the different feelings that may come and go. There may be sorrow, grief, anger—allow them to pass through you. If they become overwhelming, go back to sending metta to yourself or a good friend.
When you can, return to directing the metta phrases toward the difficult person. You can go back and forth between yourself, a friend, the reflections, and the difficult person.
As an alternative to choosing a difficult person, you can experiment with directing metta toward a difficult aspect of yourself. There may be physical or emotional aspects of yourself you have struggled with, denied, avoided, been at war with. Sit quietly, sending yourself metta. After some time, turn your attention to the loneliness, anger, disability, addiction, or whatever aspect of your mind or body you feel most estranged from. You can use phrases such as “May I accept this,” [or] “May I use the pain of this experience for the welfare of all.”
Begin [extending your lovingkindness to all beings] by sitting and extending the feeling of metta, which is friendship, caring, and kindness, to yourself. Then you can reflect on the fact that all beings want to be happy: “Just as I want to be happy, all beings want to be happy.” Then begin to direct metta to all beings, including yourself.
Extend your metta phrases: “May all beings be free from danger. May they have mental happiness. May they have physical happiness. May they have ease of well-being.” Repeat the metta phrases you have chosen and extend them to all beings everywhere, without division, without exclusion, and without end.

—Sharon Salzberg, Lovingkindness:

The Revolutionary Art of Happiness