The general consensus is that you shouldn't/can't prepare for the LNAT.
However, in the first part of LNAT you will be asked to read articles and answer very specific questions on them, where the answers are very similar, and closely related.
So one idea is to buy several newspapers (lean towards 'highbrow' - Telegraph, The Times, Guardian), ask a parent or friend to read the article and devise 10 article related questions with multiple choice answers, read the article yourself, and answer the set questions. Go through the answers and get your friend/parent to mark your score. Do this in timed conditions.
For the essay part, ask one of your teachers to set you several essay questions on general topics;
Should animals have rights?
Should animals be protected by law?
When minority and majority interests conflict, who's interest's should take precedence and why?
The one I did when I sat the LNAT was;
'It is well accepted that the right to freedom of information by way of the press is a bastion of a democratic society. The question is in what circumstances, and how, it be restricted'
Discuss
Hope this helps