Alcott employs the literary device of allusion, using John Bunyan's Pilgrim's Progress as the frame for her own novel. As in Bunyan's allegorical work, where the aptly named Christian embarks on a moral journey to spiritual maturity, the four March sisters also journey to maturity, becoming little women. To emphasize the connection to the earlier work, Alcott has Marmee give each of the girls a children's version of Pilgrim's Progress as a Christmas gift early in the novel and says she hopes they will take its lessons to heart. To underline that the moral lessons learned parallel Bunyan's book, chapters are named after the allegory, such as Vanity Fair ("Meg Goes to Vanity Fair") and the Valley of the Shadow of Death (Amy's Valley of Humiliation"). The lessons, however, are updated and made relevant to mid-nineteenth century girls: for example, in Alcott's version of Vanity Fair, Meg allows her rich friends to dress her up as a "fashion plate" for a dance, only to be humiliated when Laurie and others see her as a phony.

Little Women is also unabashedly a novel of sentiment. Sentiment, much beloved in the Victorian era, uses events and characters' reactions to them to elicit strong emotional responses from readers. Alcott works to raise reader feelings with dramatic plot devices, such as Amy burning Jo's manuscript only to almost drown when ice skating. Alcott helps us to feel Jo's pain, remorse, and forgiveness as she saves her beloved sister.

Characterization helps to sharply delineate each of the four sisters by giving them distinct traits that are different from each other: it is difficult to mistake the proper, well-mannered, and very pretty Meg from the tomboyish, disorderly writer Jo or the vain, artistic Amy from the shy homebody Beth.

Alcott also uses archetypal motifs to help us to easily identify situations in the novel: Laurie is, for example, the archetypal lonely little rich boy who needs a loving and cheerful home life while the March sisters are poor materially but rich in the intangible gifts of love, companionship, and merriment.

On a more granular level, as other answers have noted, Alcott uses such literary devices as similes, metaphors, imagery, and personification to build the vivid world of the March sisters.

Jo uses a simile when she tells Meg, "I hate to think I've got to grow up, and be Miss March, and wear long gowns, and look as prim as a China-aster!"  Jo does not want to grow up, and she especially detests the idea that she should act like a lady, as Meg wants her to.  She has no interest in looking like a "China-aster," which is a type of flower: hence, the comparison.

Jo uses another simile when she says, "'I'm dying to go and fight with papa, and I can only stay at home and knit, like a poky old woman!'"  Jo longs to be active and to go off to the Civil War with the men; instead, she is forced to stay at home, engaging in stereotypical female behavior; this makes her feel useless and without purpose, so she compares herself to a silly old woman who has nothing of importance to do (in her mind).

Metaphors are employed when the girls call Amy "a goose," and the narrator calls Beth "a mouse."  These comparisons serve to illuminate their characters: Amy puts on airs, according to Meg, and acts a bit stuck up, reminding them of a goose.  Beth is quiet and sweet and sometimes goes unnoticed because she does not draw attention to herself.  

Alcott uses an allusion when Marmee and the girls discuss Pilgrim's Progress, an allegorical text by a minister named John Bunyan.  The girls used to pretend to be characters in this text when they were children;  they would act out the adventure of the protagonist journeying toward the Celestial City—Heaven—by going from their basement all the way up to their attic.  This allusion helps us to understand the family's values and faith.

Another simile is employed when a kitten climbs up Meg's back, and it "stuck like a burr just out of reach."  The girls are particularly frustrated because they have to return to work after the holidays.  This kitten is just one more thing to irritate Meg.

In Louisa May Alcott's Little Women, the following quotes provide several distinct techniques (found in Chapter I.1 and I.2) to capture the reader's attention. First, imagery is used. Imagery is the use of specific details by the author to create a mental picture in the reader's mind.

[T]he four sisters....sat knitting away in the twilight, while the December snow fell quietly without, and the fire crackled cheerfully within.

Personification is also used in this same passage. Personification is giving human characteristics to non-human things. In the following example, the fire cannot be "cheerful." Only people are cheerful.

...the fire crackled cheerfully within.

In describing the character of Jo, the author uses a metaphor. A metaphor is the comparison of two dissimilar things that share similar characteristics. Here Alcott is comparing the young girl to a colt.

...Jo was very tall, thin, and...reminded one of a colt, for she never seemed to know what to do with her long limbs, which were very much in her way.

The author also uses a simile to create a vivid image in the reader's mind. A simile is when two dissimilar things are compared, using "like" or "as" in the comparison.

A quick, bright smile went round like a streak of sunshine.

Finally, Alcott uses an allusion, which is the reference to a famous person, place, quotation, etc. This is found in Chapter I.2 of the novel; the original quote is from the Bible in Mark 12:31—

The second is this: "Love your neighbor as yourself."

It is Meg that delivers the allusion to this familiar verse:

That's loving our neighbor better than ourselves, and I like it...

The use of a variety of techniques provides a more interesting reading experience for the reader. By creating images and quoting familiar passages, the tale comes alive and the reader is better able to not only imagine what is being described, but also to identify with the characters and become engaged in the plot development of the story.