1. Take a look at Sylvia Plath's "Mirror" & Lorna Dee Cervantes "Refugee Ship". Notice how each works with the gesture of mirror gazing, but from very different perspectives.
2. Look at yourself in the mirror. Look hard, gather in the details. Lucille Clifton's delightful "Homage to My Hips" isn't exactly a mirror poem, but it might have begun as one. You'll see that it involves a woman's unconventional response to her body, a response which rejects contemporary standards and finds her own kind of beauty and sexual energy. The same could be argued for Maya Angelou's "Still I Rise."
3. Write a poem about the experience of looking in the mirror. No rules. This need not be a description or a self-portrait although it might turn out that way (or you might start with physical description). Write what occurs to you, what comes out of the exercise of closely observing your own reflection. You can speak out of your own persona or as the mirror (as Plath does) or you can make connections to parents or siblings whose features you share (or don't share); you can write about change, growth, vanity, you name it!
A different option: If you like the persona idea, try imagining what a super model would say about her reflection. What would a professional athlete say, or a coalminer who caught a glimpse of himself right after work.