While we're talking about writing, we might well talk about writing poetry. However, much of what is said here is not restricted to poetry, but can be applied to prose.
Ideally, poetry is more compact, more intense than prose. As the late Lawrence Perrine put it, poetry speaks in "higher voltage." William Baer says further that poetry emphasizes the line, the sound of words, and compression of meaning. All of these things are true, but accomplishing them is achieved by attention to even the smallest elements. Oscar Wilde famously said he'd worked all day on a poem, putting in a comma in the morning and taking it out in the afternoon. A manuscript by Dylan Thomas includes his marginal note, "forty-two prepositions." So to achieve that "higher voltage," we have to begin with words, the building blocks of poetry. For most of us, that means taking a new look at things we already know.
We know from other writing instruction that verbs are more powerful than nouns, nouns more powerful than adjectives, adjectives than adverbs. Other parts of speech are weaker. Though they are often necessary, using many of them makes weak writing. We also know that state-of-being verbs are weaker than action verbs, and that action verbs also vary in strength. To make the strongest sentence, we should express the main idea in the strongest action verb.
My ambition is to be a poet. (Clear but weak: main idea in nouns.)
I aspire to be a poet. (Stronger: main idea in an action verb.)
I yearn to be a poet. (Stronger yet: main idea in a stronger verb.)
As action verbs vary greatly in vividness or dramatic quality, so do nouns, adjectives, and adverbs. In general, monosyllables are stronger than polysyllables, and words derived from Old English (yearn) are stronger than Latin-derived words (aspire). The poet's task is to use these degrees of strength appropriately, as Shakespeare does in Hamlet's dying request to his friend Horatio:
If ever thou didst hold me in thy heart,
Absent thee from felicity a while
And in this harsh world draw they breath in pain
To tell my story.
The soft, Latinate words of the second line give way to the hard-hitting monosyllables of the third, most of which are derived from Old English.
The placement of words is also important. The final word of a poetic line holds the most emphatic position, while the first word holds the next most emphatic. (In the previous quotation, the strongest word--pain--occurs at the end of a line.) From this we can form the basic rule: put the mos emphatic word at the end of the poetic line.
For variety, however, we can deliberately weaken that position to rush the reader through to a strong word at the beginning of the next line (the next most emphatic position):
Waking before the sunrise, she and I
Walk the woodland trails, beginning when darkness
Flows, flood-tide, and sends its somber currents
Billowing over the scarred and sullied earth . . . .
--Donn Taylor, c. 2005
The soft endings of darkness and currents weaken the line ending and pass the reader through to the strong words flows and billowing. Flows receives further emphasis by being set off with a comma.
Words that are already strong can be given greater emphasis by preceding them with weak words, a Tennyson does in the final line of "Ulysses":
To strive, to seek, to find, and not to yield.
Words or phrases can also be emphasized by putting them in a separate line, as in this satire of mine (c. 1978):
...like an experienced bureaucrat shouting "Eureka!"
as after awkward arduous hours he invents
an octagonal wheel.
Summary: 1. When possible, put the main idea in a strong verb.
2. Fine strong words and put them in emphatic positions in the poetic line.
3. Emphasize important words or phrases by putting them in a separate line.
4. Use a few weak words as possible. Use the necessary weak words to make strong words stronger by contrast.