There are a great number of devices used to develop themes. I will speak to a few of the most frequent: symbolism, metaphor, rhyme, and the soliloquy.
Symbolism is a literary device poets use to elevate their subjects. In the broadest sense, a *symbol* is anything that stands for something else. For example, Robert Burns' line, "O my love's like a red, red rose," the rose is symbolic of his love." Simply saying, "Gee, you're swell" wouldn't have the same artistic impact, you know? :)
This line also contains a device called a *similie*. A similie a figure of speech that expresses a resemblance between things of different kinds (usually formed with `like' or `as'). We can all visualize the delicate beauty of a rose, so the device aids us in understanding the beauty of his love.
*Metaphors* are also a favorite of poets. A metaphor is figure of speech in which an expression is used to refer to something that it does not literally denote in order to suggest a similarity. For example, poet Langston Hughes often uses metaphor. Consider this poem, "Dreams":
Hold fast to dreams
For if dreams die
Life is a broken-winged bird
That cannot fly.
By using metaphor here, we understand how fragile dream can be if not properly cared for.
Though many poems do not rhyme, rhyme is often used to reinforce theme by repetition of sound. It is typically easier to remember and recall themes when rhyme is involved.
Poets like Shakespeare often uses the *soliloquy* as a device to help the audience follow plot development and theme. A soliolquy lets us understand the internal dialogue of the character.
In all cases mentioned above (and many more), the devices are used to enhance, inform, and make the reading/listening/viewing experience a more rich one.