As mentioned in "first things first," the most important element of a poem is imagery.
A general rule of thumb is to strive for one concrete noun per line. A concrete noun is something you can touch. Concrete nouns will help give you a starting point for your imagery and will allow you to expand. Think of these as the bare bones of your draft.
Example:
A line without concrete imagery: "where shadows remain safest"
Here, the word "shadows" isn't concrete. It's more abstract. You'll want to avoid being too abstract in your poems, as it'll weaken your imagery.
To strengthen this line, you'll want to think of something that portrays that darker, shadowed scene without using "shadows." Something you could replace it with is "darkened stairwells." Here, the word "stairwells" is a concrete noun. The adjective "darkened" helps portray that shadowy feeling. Instead of having an abstract image of shadows, the reader now has a stronger image of an actual location (the stairwell). That image also helps evoke a stronger emotional response from the reader since dark stairwells tend to be associated with danger/fear.
(Note: All of my examples will come from poems I've written. In this example, my critique group pointed out that my use of "shadows" was too abstract. I have a tendency to use "shadows" quite often in my writing, and I'm slowly starting to break that habit.)
