One function of line length is to act as a score for how the reader is to approach the poem.  Long lines flow and shorter lines read choppier.  Long lines are read a bit quicker, short lines slow the pace. To gain a better understanding, we need to have some knowledge of how punctuation and the breaks influence the reading.  Take for instance this piece of prose.  If we were to read this as poetry, we automatically pause at line breaks, commas, and especially at end punctuation.

The geese fly low to our left, (pause) the gulls sit and appear to (pause) 
shiver (pause)
but the ocean doesn’t look so bad. (pause) 
The wild seas of the (pause)
last few days have flattened out some in the wind. 

                                                          Peter Kaminsky,The Moon Pulled 
                                                               Up an Acre of Bass (2001)

Try to shape your breaks so that they occur naturally, where you would like a pause in the reading.  Consider how the line break of to/shiver interrupts the thought, leaving ‘shiver’ on the next line out of context.  The next break the/last interrupts the grammatically structure of the sentence, and since the last word of the line is slightly emphasized by its position, ‘the’ is in a place of emphasis.  

In free verse, there are several ways to effectively break the lines; breath length, grammatically, and to emphasizes a particular word or to embed a double meaning.  To break the prose lines more poetically, we might do so at the natural grammatical pauses and to isolate a single image per line:

The geese fly low to our left,  (pause)
the gulls sit and appear to shiver  (pause)
but the ocean doesn’t look so bad.  (pause)
The wild seas of the last few days   (pause)
have flattened out some in the wind. 

Try reading it with the lines broken into smaller increments.

The geese fly low  (pause)
 to our left,    (pause)
the gulls sit  (pause)
and appear to shiver  (pause)
but the ocean  (pause)
doesn’t look so bad  (pause)
The wild seas  (pause)
of the last few days  (pause)
have flattened out some  (pause)
 in the wind. 

The line breaks act a bit like speed bumps and the pace of the sample poem with super short lines is very stilted.  There is however, a double meaning now imbedded into line two, it can now be read with both the line preceding and following it, as if both the geese and the gulls are ‘to our left.’ Short lines can be used effectively to reinforce the meaning of a poem, providing a sense of difficulty to the speaker’s emotional or mental state.  Be aware that a single word on a line receives much emphasis, and that too short lines can make a poem seem amateur and overly dramatic.  

Compare the speed of this version broken according to where you might breathe while reading: 

The geese fly low to our left, (slight pause) the gulls sit and appear to shiver (pause/breath) 
but the ocean doesn’t look so bad. (pause/breath) 
The wild seas of the last few days have flattened out some in the wind. (breath)

You should feel a sense of rushing to get to the end of the line in time to breathe.  Without as many line breaks, there are fewer pauses and the pace is quickened.  

The lines can also be formed by the sentence; Marvin Bell’s recent The Book of the Dead Man  is almost completely composed with lines corresponding to sentence length.  To make a poem work with all end-stopped lines, there must be variety in the structure of the individual sentences.  (Follow the link for sample pages from the book and to get a taste of the range of possibilities in sentence structure.)

When a line is enjambed, or the sentence wraps into the next line, there is an opportunity to add a double meaning. William Carlos Williams uses this technique in his poem Burning the Christmas Greens.

 . . .we brought branches
cut from the green trees

to fill our need, and over
doorways, about paper Christmas
bells covered with tinfoil
and fastened by red ribbons. 

As we read we are always tracking for meaning so when we find a line like "to fill our need, and over” the first ‘meaning’ that we see is that the need is filled and over (filled).  Of course, as we read on we understand that the “over” is in relationship to the doorways, but the first concept has already been processed.  This “overfilled” concept is reinforced by the next line break, see Williams has paired ‘paper Christmas’ and ended the line. He again places a subtle emphasis on Christmas as ‘paper’ or quickly passing away, highlighting the wastefulness of cutting the branches.    

As you revise, look at the poem as a whole consider what pace you would like it read and then shape your lines accordingly.   It helps to read the poem aloud and overemphasize the pauses at the line ends to see if they make sense. Try to avoid extremely short lines that feel like declaiming and remember that few words are important enough to be on a line alone.  Take hard look at the words that end the lines. 

These are the words that ‘sink in’ during the pause, make certain that are strong and add weight to important words & ideas in the writing.   

Once you have shaped the overall line length, look again for any secondary meanings that a line break could draw out.  In free verse, while not bound by lines of a particular metrical length, it is important to keep in mind that even shaping the length of the lines is a technique, another way to make the ‘form’ of the poem support the content.