Find the person or thing the author feels most negatively about. Try to find a word or phrase that expresses how they feel. For example, in the LA from the exam last year you might decide that the author feels *most* negatively about ebooks; you might then decide that her major negative opinion is kind of more like doubt and scepticism. Bang - that's your first BP. Go through and just hit "doubt and scepticism" hard, focusing only on that. What is every bit of the piece that gives you doubt and scepticism about ebooks, and how does it?
Then you find the person or thing the author feels most positively about. Try to find a word or phrase that expresses how they feel. For example, in the same piece you might decide she feels most warmly towards reading as an activity; you might then decide that her major positive opinion is delight and awe. Bang - that's your last BP. Go through and find everything that communicates "delight and awe", and explain how.
Then you just need one or two other things she has an opinion about (opinion = tone). They go in the middle. For last year's, you might decide that BP2 is concern for children, for instance. Go hard on 'concern' for the whole para.
I find that most often people lose steam when they're too general and descriptive - after an intro and one paragraph, you've kind of described all there is to describe! Focus on the contention for your intro; for your BPs, worry more about people and things the author has an opinion or feeling about. Then just prove that ONE feeling the whole paragraph.
