Beautiful girls have long driven men to poetry in an attempt to describe them with words. Because so much of poetry deals in descriptions, a wide array of poetic devices are perfect for describing female beauty. When properly combined, they can create a richly visceral verbal image of a lovely girl.
Denotation and Connotation
The shorter a description, the more important is each word --- and when you're trying to sum up a girl's beauty in a single word, finding the perfect word is imperative. The word "beautiful" has many synonyms, most sharing the same denotation, or basic meaning. Their connotations, however, or subtle distinctions and shades of meaning, vary widely. The word "gorgeous" implies sumptuousness, for example, while "ravishing" implies a sort of sexual enchantment.
Metaphor and Simile
One way to describe a beautiful woman is to compare her to something else. Comparisons with the beauty of nature have long predominated, famously exampled by William Shakespeare in his 18th sonnet, which begins "Shall I compare thee to a summer's day?" Simile is saying that she is like something, while metaphor skips over the in-between language and simply states that she is the thing described.
Synecdoche is a poetic term for treating a part as if it were the whole, paying attention to a specific detail rather than generalities. Rather than trying to describe her entire beauty, limit yourself to her eyes or lips. Such description can and should include other poetic devices, such as metaphor and overstatement.
Overstatement, also known as hyperbole, is exaggerating something for emphasis --- in this case, your description of the girl's beauty. Embellishing your depiction can help the reader imagine what she is like, and if done properly, will be recognised as overstatement and not mislead. Be careful not to take it too far, however. Shakespeare's 130th sonnet makes fun of poets who unskillfully use overstatement, with the following lines:
"I love to hear her speak, yet well I know
That music hath a far more pleasing sound;
I grant I never saw a goddess go;
My mistress when she walks, treads on the ground."