When analyzing any of Shakespeare's sonnets, the key to discovering the use of literary elements (or techniques) is to first identify the subject of the poem, and then ask what the author is saying about this subject, and how.  And, simply because this is Shakespeare, you can almost always expect to find an overarching metaphor at work in the piece.The subject of Sonnet 55 is beauty.  In each quatrain, nature imagery is used to define beautfy (and show the speaker's lament) through compare and contrast.

Throughout the sonnet, beauty is measured against mortality.  These two abstract concepts are made metaphorically concrete by personifying their comparative power against elements found in nature (brass, stone, earth, sea, rocks impregnable, gates of steal versus "summer's honey breath").  Mortality has the power to destroy (or change) all of these things, but beauty, whose strength is no more substantial than that of a flower, is powerless.

The use of nature imagery and metaphor (through personification) is complimented by Shakespeare's additional use of irony and rhetorical questions.  The questions throughout the poem seem to prompt the idea that beauty cannot possibly win in the fight against time.  They are considered rhetorical because the speaker answers them as if their is only one correct response (How shall summer's honey breath hold out? It can't!).

But the final couplet at the end of the sonnet holds the final answer, which is an ironic twist:

O, none, unless this miracle have might,
That in black ink my love may still shine bright.

It is through words that beauty is preserved, and in this way, it will easily outlive, out-power, and outlast mortality or time.

Understanding the poem broadly should help you to go back and analyze it quatrain by quatrain, or line by line even, to extract smaller and more detailed examples of literary elements.