A few words on  how to write a good essay 
Essay carries 200 marks, sufficient to shuffle your rank in  the final list. Those students who have one of their optional as History or  Geography or Sociology or Pub add or Psycho etc., definitely have an advantage  over others in writing essay. Science background students generally lack in  this. 
Here I can share my experiences. I have got only 85 marks in  essay in CSE-2008 and that figure rose to 106 in CSE-2009. I can give a few tips  on this:
1.       Choosing the right topic: When you are  sitting in the exam, there it’s the most important task and most difficult too.  Reason is that you do not have much choice. Only 5-6 topics are mentioned.  Moreover there wording is vague and quite general in nature. So choosing a right  topic becomes very essential. 
The main idea that one should keep  in mind is that never to choose that topic which is clearly giving an advantage  to a particular optional students e.g. let us say there is an essay on Indian  Freedom Struggle: Process complete or not. Here I am just taking an example, so  do not go into its details. The topic from its name itself is giving a fair  indication that it’s a topic which will be best dealt by history optional  students. If I having physics and punjabi lit. as optionals will try to do this,  then I am having only that much knowledge of history which is essential for GS,  which is virtually not more than 5% of that of the other person with history as  optional. I hope that this point is clear.
Another thing is choosing that  topic for which you have sufficient facts and figures in your hand. Try to  substantiate as much as possible with statistics. It’s the best way to answer  even in GS too.
Also try to choose that topic on  which your ideas flow smoothly while writing the essay. Flow should not be  turbulent but a smooth one, like a river.
2.       Think for almost 30-35 minutes on that topic.  I am telling you the optimum limit of 
this. 
Never dedicate too much time on  thinking otherwise you will be left with very short time to write. Use pencil to  accumulate all the ideas that come to your mind for that topic and write them  down on the last page of the sheet. After jotting down the points, try to make  their order from first to last.
While thinking, think along these  lines: Historical, Legal (related to our constitution), Social and Economical  (related to market), International.
3.       Your start must be very effective. You can  begin with a quotation or a question or a short story or anything which arouses  curiosity.
4.       Do not make headings while writing. This is  the mistake I committed in CSE-08. Of course, you have to make paragraphs but  without headings or underlinings.
5.       Never miss any edition of Yojana. I started  reading Yojana regularly after I got less marks in essay last year. Thereafter I  did not miss any edition. Kurukshetra can also be followed but its approach is  more rural and its more inclined to agriculture. But Yojana covers each and  every thing. Try to remember the facts and figures from it.
6.       Always write whatever is the latest. Try to  follow the latest economic survey. Some basic facts should be on your finger  tips like our irrigation potential, roads density: rural and urban, Rail network  length, SC and ST population, Drinking water stats, Primary secondary and  tertiary health care centres etc. (apart from deficits that you have to remember  for GS economics).
7.       Length of essay does matter. Try to cover the  essay from each and every angle. Never show only one side of the picture. Try to  give the best possible answer in a balanced manner. But obviously never try to  write just for the sake of increasing the length. Whatever you write must be  logically related to that topic.
8.       End of essay should also be equally  effective. Believe you me, even if your body of essay is not that great,  a perfect end will do the job for you. Try not to repeat the same words. Use  good synonyms and show the best of your word power while concluding. But beware  of using jargons at the same time. Here again, you can use some quotation or try  to give the answer if the start was from a question.
9.       Practice before the actual exam in strict exam conditions. 
Try to write essay on all the possible topics. But nowadays the trend is that they are not putting those topics which are more expected. Like I was preparing an essay on global warming since last 2 attempts but they did not ask despite this was such a hot topic. Essay on our education system was also most expected, because of the changes that are proposed in both the school and univ. education system but it also did not figure in the list of five.So, try to write essay on all the topics which  are in news and also which are not that important. You can take such topics from previous 10 year essay papers.
I hope these ideas will help you while writing essay.
Thanks.
May Waheguru G make U an IAS officer.