Civil Services Examination preparation is a long drawn affair and sincerity shown in preparation would certainly reflect in your end result.
But, don’t get confused; rely on standard text-books only as no coaching institute material can replace these as resource. As you still have two more years in between, you should stay in touch with what you have read and keep on topping up information with current developments.
Do not overload yourself instead of reading books time and again, assimilate the important information in shape of concise notes and keep revising them as and when you come across some useful input. This will help in staying in touch as well everything would be crystal clear as you would know things inside out.
Answer-writing and getting checked by some expert or senior who could get you the correct advice would prove to be of immense help. Answer writing skills would improve by adding some more contemporary information and covering all possible direction and perspectives and finally, this practice would help you in carving holistic answers.
If you have finalized your optional subject so, why to wait, commence preparation for that as well. Make such a strategy that by December 2015, you should be well prepared and after that only revision, updating and practice remains top-of-mind.
So, keep preparing and maintain consistency in study-plan.
All the best!