Analysing a poem is most importantly demonstrating how certain features and techniques serve to emphasise a poem's subject and themes. Make sure this is at the back of your mind throughout your analysis and essay writing and that at the end of each point you LINK this in to your writing. This will make your essay strong and comprehensive.
When analysing, it is important to be methodical. Read the poem once in your head, then once out loud. By doing this you get a sense not only of the poem's meaning but also of its rhythm and rhyme. Think about the form of the poem, how the poet has chosen to structure it and how this draws out a theme. For example if the poem's subject is water and the lines are broken up all across the page, this could suggest the fragility and transience of nature in its ever changing state. When thinking about rhythm and rhyme, make sure you notate the rhyme scheme (e.g. 'abab') down the margin and take care to mark any line breaks, caesura's, end-stopping and enjambement. They are the core of the poem's flow, giving emphasis to certain words. It is then a good idea to highlight key words, language that is striking in any way and that correlates to the themes which you have started to notice. Analyse this language by noting its sounds, position, what imagery and tone it is creating. Remember to use technical words in your essay such as onomatopoeia, alliteration, simile, metaphor and so on. It adds depth to your essay and shows that you understand what you are talking about. Finally, remember to link each point to a theme in order that you answer the question!
A bit of revision adivice- if you are given a portfolio of poems for the exam, it is imporant to make sure that you do not try to guess which one(s) you will be asked to write an essay on. Try to familiarise yourself with each poem as much as you can. Once you have a fully anotated poem, you could print out several plain copies and note as much as you can remember on the new copy. Then compare and see what you have forgotten. This repetition, although a little tedious, is bound to make the information stick and provides a challenge. It also means that when you get into the exam, you can do this to the poem you are given immediately, and with minimised stress.