"Upon Julia's Clothes" is, for the most part, written in a meter called iambic tetrameter, and it's a meter Herrick used quite a bit in his poetry. Each line consists of four (tetra-) iambs (a unit that contains an unstressed syllable followed by a stressed syllable). Don't sweat, stalwart Shmoopers, we'll break it down.
As an example, take line 3:
That liquefaction of her clothes
In short, we've got four stressed syllables in the line, each of which is preceded by an unstressed syllable. So the line goes a little something like this (ahem): da-DUM da-DUM da-DUM da-DUM. (Applause. Applause.)
Okay, we've got that down, right? Not so fast. Although iambic tetrameter is the dominant meter in the poem, most of the lines contain examples of other types of beats, just to throw us for a loop every once in a while.
So, for example, the first foot of line 2 contains two stressed syllables (called a spondee) before resorting to iambs the rest of the way:
Then, then methinks how sweetly flows.
The most interesting metrical variation (when the poet switches up the poem's structure) in the poem, however, occurs in the last line: O how that glittering taketh me!
The first foot is a trochee (DA-dum), the second an iamb (da-DUM), but the third foot is something called an anapest (da-da-DUM). The use of an anapest makes the word and the line "glitter" or twinkle in a rather charming way. That little extra syllable adds a little extra sparkle. We told you Herrick had style!