Each stanza in "Before You Were Mine" contains at least one example of alliteration. Sometimes this links two or more words or concepts together, and other times it works to bring the poem's images to life.
Take the repetition of voiced and unvoiced /th/ sounds in the second stanza:
I’m not here yet. The thought of me doesn’t occur
in the ballroom with the thousand eyes
The alliteration between the two "the[s]" and "thought"/"thousand" is spread out, but the pairing of the phrases nevertheless creates a contrast between two worlds—pre- and post-motherhood. Soon, "the thousand eyes" of potential young loves at the ballroom will give way to "the thought of me [the speaker]"—the constant attention needed to take good care of a child.
Later in this stanza, alliteration links "mine" with "Ma," setting up another contrast between the two people with a claim to the speaker's mother—the speaker herself during childhood, and the mother's own "Ma" back when she (the speaker's mother) was young.
Line 12's alliteration marks another significant moment in the poem, coinciding with the speaker's first mention of one of her actual memories (the scenes in the first two stanzas predate her existence):
I remember my hands in those high-heeled red shoes, relics,
These shoes are an important symbol that represent the mother's lost youth. The /h/ sounds are breathless, suggestive of energetic movement (like dancing the cha-cha!). Red is a color associated with lust and passion, and the alliterative pairing with "relic" here underscores that fact that such lust and passion are now things of the mother's past.
In the final stanza, the poem dials up its alliteration. This mirrors the assertion that the mother's "glamorous love lasts"—she continues to "sparkle and waltz and laugh," perhaps through her love for her daughter. The alliteration in this stanza through "stamping stars," "somewhere [...] Scotland," "before [...] born,"and "love lasts [...] laugh" adds a sense of vibrancy and energy to the poem's final image.