Poems are usually short, so there is no room for anything extra, or anything that might confuse a reader. You can think about literary devices, but don't force them. If you want to use a simile, a metaphor, alliteration, or onomatopoeia, go ahead. But don't put in devices just because you think every poem has them. The same goes for rhyming.
Some people can bang out a poem in perfect form and have it rhyme perfectly the first time.
But, don't force it. Most poetry doesn't rhyme. And you have to be careful because words that rhyme too well make your poem sound really fake. Once you get the form down, and you have said and described most of what you want, you can work on the words of the poem. A good way to play with the words is to do poetry corruption, the game we played in class where I wrote real poems on the board with various words left out.