If you read the chapter about the Lady of Shalott in King Arthur and his Knights of the Round Table, it really helps you understand this poem better.

The simile is a comparison of two unlike things using the word like or as.  The trick here is not assuming because these two words are used, a simile is there.  In this poem, the word as is used often, however there is only one true simile where as is used.

Here are the similes:

1.

The gemmy bridle glitter'd free,
Like to some branch of stars we see
Hung in the golden Galaxy

In this stanza Tennyson is comparing Lancelot bridle to a constellation in a golden galaxy.  Clearly, he is trying to help us see how shiny the bridle must have appeared to the Lady of Shalott as she watched in her magic mirror.

2.

The helmet and the helmet-feather
Burn'd like one burning flame together,

This simile again demonstrates how striking he must have appeared.  The next simile appears right after the description of the helmet and helmet feather:

3.

As he rode down to Camelot.
As often thro' the purple night,
Below the starry clusters bright,
Some bearded meteor, burning bright,
Moves over still Shalott.

This comparison compares Lancelot's ride through Camelot to a meteor streaks through a purple sky.  Note the word as is used to create this comparison.

4.

And down the river's dim expanse
Like some bold seer in a trance,
Seeing all his own mischance --
With a glassy countenance
Did she look to Camelot.

The Lady of Shalott, realizing that Lancelot loves another, prepares to die knowing that if she leaves Camelot the curse will kill her.  Knowing she will never have Lancelot, she does so anyway.

What are four similes used in "The Lady of Shalott"?

Let us remind ourselves that similes are comparisons established between one thing and another indicated by the word "like" or "as." They are very different from metaphors, which likewise compare one thing with another but they assert the comparison directly without the need for "like" or "as." If we view the poem with this in mind, we see that the poem does not use any similes at all until Part III and the arrival of Sir Launcelot on the scene. Note the way that similes are used to describe his appearance and imbue him with a vitality and life that heightens the contrast to the dull existence of the Lady of Shallot:

The gemmy bridle glittered free,

Like to some branch of stars we see

Hung in the golden Galaxy.

Likewise we are told in the next stanza that:

The helmet and the helmet feather

Burned like one burning flame together

The focus on comparing him to a flame and then to stars glittening at night combine to make him attractive and distinct, full of life and vibrant. His appearance is then compared to a meteor in another simile:

As often through the purple night,

Below the starry clusters bright,

Some bearded meteor, trailing light,

Moves over still Shallot.

Lastly, a simile is used to describe the Lady of Shallot herself in Part IV of the poem and her appearance as she looks down to Camelot in her boat:

And down the river's dim expanse

Like some bold seer in a trance,

Seeing all his own mischance--

With a glassy countenance

Did she look to Camelot.

This simile indicates the way in which the Lady of Shallot is contemplating her own doom, now that she has made the curse come upon herself through her actions and embraced life.