In writing poetry, one of the main things to focus on is being exact. In other kinds of writing you can use three or four sentences to express a thought, but in poetry you have to express the same thing in three or four words... so picking the right ones matters a LOT.
When you write the first draft of a poem, it is okay to use whatever words come to mind, but after that... drill down on each word... does it need to be there? Can it be cut out and still retain the overall meaning? Is there a better word to express this? Are you using multiple meanings of this word? If so, how can you show that in the rest of the poem? Focus on being as conscise as possible, without losing the idea. Even in rhyming poetry, make sure you aren't throwing in things that you don't need just to make the rhyme. Every word matters.