In his closing discussion on loving-kindness, Buddhaghosa asks: "What is the proximate cause of loving-kindness?" The answer is the observation of lovableness in the person to whom you are attending.
Bring to mind right now someone whom you find lovable. It could be a person you have a romance with, or a child, or a dear friend, or a great teacher--someone to whom your heart would leap like a deer in the forest if this person were to walk through the door, someone whose presence is so lovable that a gladness arises on seeing him or her. If you can sense that in a dear friend, then try to seek out the lovableness of a neutral person. Then, finally, when you break down all the barriers, see it in a person who has done you injury.
It's a great key if you can seek out something to love, even in the enemy. Bear clearly in mind that this does not endorse or embrace evil. The crucial point here is to be able to slice through like a very skilled surgeon, recognizing vicious behavior that we would love to see annihilated as separate from the person who is participating in it. The doctor can be optimistic. A cure is possible: the person is not equivalent to the action or the disposition. Moreover there is something there that we can hold in affection, with warmth. That really seems to be a master key that can break down the final barrier and complete the practice.
One way of approaching this is to look at the person you hold in contempt, and try to find any quality he might share with someone you deeply admire and respect. Is there anything at all noble to be seen, anything that would be akin to what a truly great spiritual being would display? Focus on that: There is something there that you can love. The rest is chaff, that hopefully will be blown away quickly, to everyone's benefit. It is as if you could see a little ray of light from within, knowing that its source is much deeper than the despicable qualities on the outside. That light is what you attend to. (p. 112)
--from The Four Immeasurables: Practices to Open the Heart by B. Alan Wallace, edited by Zara Houshmand, published by Snow Lion Publications