Bhai, 'how to score good marks?' is really very difficult question to answer.
If you had asked how to write good essay, then might be you already have got proper response. Please accept my appology that I am also not aware how to get good marks. Lets have a hope that some 'examiner' may read this post and tell us how we can get good marks.
Your second question is about the issue of heading/ no heading.
Well! if you see essays written by world's distinguished essayists like Becon, Devid Hume, John Locke, Bertrand Russell etc., you shall find that all these prominent essayists never have used headings in their essays.
But if you are asking the impact of heading/ non-heading on MARKS, then please again accept my excuse because I am just a candidate and not examiner. But I am also eager to know the answer to your important questions. So lets see may be some examiner really happen to read this post and tell us the right answer.
Thanks!