I love love poems (and love songs), and have since I was a tortured teenager--in love (aren’t we all at some point?) with a someone who would not love me back.
Let me clarify. I love good love poems, that is. Bad ones are the worst kind of poem out there, even worse than bad political poems.
What makes a good love poem? Pretty much what makes a good poem in general: specific images (your images, not Hallmark's), surprising rhymes (if you’re rhyming, that is), and a creative take on love and/or the beloved. If you've been raised on red roses, you'll need to step out of your comfort zone--it can't be all moon and June. Sometimes, in fact, love is hate (see my poem titled "Love," featured in The Best American Poetry 2009).
Examples? I don't think I'm alone in my admiration for Shakespeare's love sonnets, especially the one that begins "That time of year though mayst in me behold / when yellow leaves or none or few do hang / upon these boughs which shake against the cold / bare, ruined choirs where late the sweet birds sang." And John Donne's "The Sun Rising" http://www.luminarium.org/sevenlit/donne/sunrising.htm.
Edna St. Vincent Millay’s “Love is Not All” is another one of my favorites:
Love is not all: it is not meat nor drink
Nor slumber nor a roof against the rain;
Nor yet a floating spar to men that sink
And rise and sink and rise and sink again;
Love can not fill the thickened lung with breath,
Nor clean the blood, nor set the fractured bone;
Yet many a man is making friends with death
Even as I speak, for lack of love alone.
It well may be that in a difficult hour,
Pinned down by pain and moaning for release,
Or nagged by want past resolution's power,
I might be driven to sell your love for peace,
Or trade the memory of this night for food.
It well may be. I do not think I would.
Memorizing image-rich and ear-delighting poems like these is a great way to alert and prepare your brain for the kinds of images and rhythms you’re going to have to drum up and/or dig down deep for when you sit down to write your own love-induced (love-wracked?) verse.
As you’re composing, keep reading the best love poems you can get your hands on. Search out the oldies but goodies (Thomas Campion, William Blake, John Keats, Robert Herrick, Anne Sexton, William Stafford . . .), but you must also your contemporaries. Kary Wayson writes incredible love poems,; so does Olena Kalyatiak Davis. Adrienne Rich's love poems taught me how to write about sex.
A good anthology of love poems? When I was trying to find poems to be read at my wedding, I came across Robert Hass’ Into the Garden: A Wedding Anthology: Poetry and Prose on Love and Marriage. There I found some of the world’s best love poems by Sappho, Whitman, Dickinson, Milosz, May Sarton, Sharon Olds, James Wright, and WC Williams.
And then, off in its own galaxy of love, is Pablo Neruda's "I do not love you."
As you become more acquainted with the territory, your attempts at writing love poems will likely improve. You might write a sappy poem or two, but if you've done your homework, if you shared what makes your relationship and/or your beloved unique in your own true voice, you'll end up next Valentine’s Day with a love poem you can share not only with your beloved, but with the world.