We've generally been taught that to achieve an A* (which as another poster said, is a conceptualised interpretation), it is important to go through the main PILL TVR (purpose, imagery, language, layout/structure, tone, voice and reader response) points, comparing and contrasting both
within the text, and for poetry,
across the poems. Link these points back to the purpose (shows your understanding), then in turn back to the question.
Other useful elements to include:
-an understanding of the social-historical context and how this links to the author/poets purpose
-evidence that you understand alternative interpretations of the poems or of certain language/imagery features
-close language analysis of phrases/word choices that have particular relevance to the poet's purpose/the poem's effect
-some knowledge of literary terminology and specific techniques; this in itself won't get you a higher grade, but if you can identify the techniques used then explain them and analyse their effect, you're signaling to your examiner that you're working at higher levels
-fluent, cojent writing > lots of discourse markers to "signpost" your essay, make it more fluid. "This connotes/symbolises/conveys/imparts/evokes/expresses" - variety of vocabularly keeps your essay from being repetitive
-use the question only as a starting place, and expand your answer by thinking around it - keep your points relevant enough that they can be taken back to it with a simple linking sentance, but don't be afraid to go a little further than the immediate scope of the question would suggest - i.e, if you get a question on how feelings of love are used in Heaney and Clarke, don't be afraid to talk the effects of other emotions, the effects of juxtaposition of emotions, the contrasting effect of the absence of emotions/love etc.
FINALLY - do not underestimate the importance of the accuracy marks in achieving the top grades; I don't know which exam board you're with, but for mine they're worth a third of the marks (9/27 for each assessed task). Use a full range of grammar competently and assuredly throughout, and leave ~ 5 minutes to check through your work (particularly basic spelling). Its amazing what a difference that can actually make
Hope this helps =)