Open-form poetry rejects the organization and structure found in traditional poetry such as sonnets and haiku. Open poetry — including chance poetry, Surrealism, and free verse — use experimental techniques that blast open the possibilities of words on the printed page.
The open form is a form. When you use the open form, you start to impose on your poem — and yourself — all sorts of rules. You won't know what they are until you get there, however, because each poem is its own form.
Think of open-form poetry as a way of thinking — an especially intense awareness of every single aspect of the poem, from subject and tone to music and rhythm, from the physical shape of the poem to the length (in space and in time) of the lines, from the grammar you use to the parts of speech.
When you write an open-form poem, try to be very conscious. Everything in the poem, every feature, every aspect, must have a reason for being there. Be conscious of the following:
Economy. Cram as much energy as possible into each word. Cut everything that doesn't absolutely need to be there.
Grammar and syntax. Are you always using complete sentences? Well, that's fine — but you could also do it another way. Decide whether you have a reason to write in complete sentences for this poem. If you can come up with a reason, fine. If not, consider alternatives — bursts of words, single words, word fragments. And who says you have to use "proper" grammar? Or punctuation? Try breaking a few rules, if that improves the poem.
Parts of speech. Some teachers say you shouldn't use adjectives or adverbs; they prefer nouns and verbs instead. That's an excellent starting point: Use only the words you need. If all you're doing is prettifying something, forget it. Use adjectives only when they're surprising ("your green voice"), contradictory ("aggressive modesty"), or give information the reader simply can't get elsewhere ("It was a Welsh ferret" — how else would we know a ferret was Welsh?).
Rhythms. Look at the rhythms in your lines. Does the rhythm of the line contribute to its meaning? Anything sing-songy? If so, is it good that it's sing-songy?
Often, open-form verse falls into iambs (a group of syllables consisting of an unstressed syllable followed by a stressed syllable, as in "alas!") and dactyls (one stressed syllable followed by two unstressed, as in "penetrate"). Don't let this happen unless there is a reason for it.
The physical lengths (the number syllables and the actual length) of the lines you use. Avoid falling into exactly the same lengths. Every length should have a reason behind it.
The length (in time) it takes to read each line aloud. If each line takes about the same number of seconds, figure out whether there's a reason for it. If there isn't, consider other shapes and lengths.
Line endings. Poets realize that line endings carry a certain emphasis or pressure. Your lines should end where they end for some reason. The way a line ends — where, and after what word or punctuation mark — should be the best way to end. Do you want a pause there? What's going to happen when your readers go to the next line? Something unexpected? Some surprise?
Read a lot of open-form verse, and you'll notice that poets use a great deal of enjambment, winding the words around the ends of lines in gorgeous and meaningful ways.