I never prepared separately for essay because if you are well versed with your GS syllabus, then you can easily attempt an essay on any topic.
However, your essay should be well structured (in terms of introduction, conclusion, thought flow, exploration of dimensions and original thinking).
Further, it is estimated that essays are checked by people from literature background, so your grammar and language also matters a bit.
Finally, since most of the people select the same topics for essay (and may even write the same stuff as well), you may need to differentiate your essay and make it attractive. You may do it with the help of relevant quotes, suitable facts and figures, some anecdotes, diagrams and illustrations and so on. There are no fixed rules of writing UPSC essays. Try to be as innovative as possible.
Best of Luck!