What similes does Romeo use to convey Juliet's beauty?

A simile is a comparison of two unlike things using the words like or as. The comparison tends to enhance the qualities of one of those things. Immediately after seeing Juliet at Capulet's party in Act I, Scene 5, Romeo compares her beauty (it is definitely her looks he is speaking of here because he has yet to meet her) to a precious jewel:

It seems she hangs upon the cheek of nightAs a rich jewel in an Ethiop’s ear

This comparison needs a bit of explanation because one might not understand what Romeo is speaking of. It was probably common in Renaissance Italy for men from Ethiopia to act as traders and to visit Italian cities. Romeo may have seen just such a man, or maybe read about them. These Ethiopians were often wealthy and would have adorned themselves in elaborate jewelry such as diamonds, emeralds or rubies.
 
Later, in Act II, Scene 2, Romeo once again uses a simile to describe Juliet. Here he may also be speaking of her personality and not just her beauty because he has spoken to her briefly in the waning moments of Capulet's party. He compares her to an angel coming down from heaven as a messenger to bring happiness and light to Romeo's life (which, by his own admittance, had been darkened by his unrequited love for Rosaline):


O, speak again, bright angel, for thou artAs glorious to this night, being o’er my head,As is a wingèd messenger of heaven



This comparison to a "bright angel" is very in much in keeping with Shakespeare's recurring motif of light and dark which pervades the tragedy.