If you have a commerce degree, it's the best subject to choose as the syllabus matches well with your degree syllabus for both the papers.
For CA students your CA PCC/IPCC material is very useful. For the rest of the students too commerce is a good subject as the syllabus is static in nature but you may require taking coaching for the basics. The good thing is it's a very scoring subject.
If you are one of those that like solving numerical, you are at an advantage because the paper contains about 150 marks worth numerical questions. Do remember though that UPSC numerical are very unpredictable and requires high levels of competence. But again that's true for the subjective answers too.
Commerce is a time consuming paper to prepare because the syllabus is bit lengthy. It requires a lot of reading from many books and sources and then you need to integrate them into the context of the syllabus. But that's good, because the more notes you write from different sources the more practice you get of writing your answers in the mains exam. You can take up some Test series for practice as well.
One advantage with commerce is that normally two weeks gap for revising the syllabus is available. This is an added advantage as compared with other optional subject.
Paper 1 consists more of practical questions so scoring above 200 is fairly easy. The key for scoring more marks in paper 1 is PRACTICE. Theory based questions should also be prepared carefully as compulsory questions are theory based, even though some current affairs related questions are also asked now a days.
If you carefully see the syllabus of Paper 1, you can notice that the level of Paper 1 is equivalent to CA (Inter or IPCC) or B.Com (Hons), hence books of that level are sufficient to cover the entire syllabus and one should not read the entire books prescribed. Rather only the UPSC syllabus topics should be covered from those books.
In the exam hall patience & calmness holds the key since there will definitely be unpredictable & unexpected questions. You've to smartly avoid them in Choice and for compulsory questions don't beat around the bush if you don't know the answer. Rather invest the time in known questions & numerical. Even if 25-40 marks questions are unanswered, still you can achieve 140 easily. Try to attempt more numerical than theory.
For paper 1, exam day revision is difficult, so make chapter wise notes of important formulae and adjustment which are tricky especially for Account, FM and Cost. For tax, more details may be required.
Accounting Standards and presentation for numerical should be taken care of properly with working notes. No need to practice again & again. Just mark the important adjustments/questions and refer them only in subsequent revisions.
References: JR Monga & DS Rawat(For AS) OR Material of CA PCC.
Emphasis should be given on theory equally.
References: Naseem Ahmed (A very good & simple book to cover entire topics, selected topics only), Maheswari & Mittal plus theoretical part from either book itself or some compilation like CA PCC scanner. Also material of RANKER's CLASSES (RC) provides very good compilation of numerical and theory. OR Material of CA PCC can be referred.
In depth study initially, is important and later focus on basics and specially marked sections need to be covered again. Theory is important as overall questions are of graduation level only barring some exceptions.
References: V.K. Singhania or Girish Ahuja or CA IPCC by Ajay Jain
References: Students' Guide to Auditing- Aruna Jha (Taxmann) for Selected topics. CA PCC notes for relevant topics or Compilation from RC.
Following is the link for an Income Tax book by Bhavin Pathak. This book is written in a wonderful way and one should download for covering Income Tax part from this book
FM has been the most difficult section in recent times. Due to a wide range it is better to simply understand the different models/concepts/terms
References: RP Rastogi OR CA PCC notes plus relevant topics from CA Final can also be seen. For overall syllabus Text & Problems by Khan & Jain is sufficient.
This section is highly unpredictable so do not to waste much time on it.
References: Relevant Chapters from Indian Financial system by Bharati V Pathak or any other book and newspaper cuttings provided by RC. Keep track of steps/initiatives by RBI, SEBI, IRDA etc. Indian Financial System- M.Y. Khan or selected topics from Financial Institutions & Markets- L.M. Bhole
Above books are sufficient to cover the entire syllabus for paper-1 and one need to practice regularly to get a hold over this paper. One can see its own source as well for the above syllabus as for his/her own comfort like CA books for covering some of the syllabus.
Preparation should start with a general reading & developing the conceptual clarity with subsequent reading focusing upon important definition by authors/thinkers, establishing links with other topics/chapters and current corporate practices.
After reading the definition & basic explanation try to think before studying in detail.
Rankers material and class notes for paper 2 together with good reading of basic books mentioned below should suffice.
Note making is very important here wherein diagrams, verbatim defintions (by thinkers),examples & only core points should be noted without explanation. Topics from all sources should be integrated at one place. Important quotations should be memorized
In the exam hall take care of a few things. Choose very carefully the compulsory questions as little knowledge of every question shall be there but you should be sure which one you know better. Careful reading of question is a must to know the operative part. Make use of all chapters and topics in your answers but basic tone of answer should be maintained. For example; for HR answers, focus more on such dimensions and less of OB etc. Also avoid GS type and general answers so as to score better.
For long answers first introduce the topic with quotations or a general explanation, then follow it up with an exact definition. Then explain topic by establishing links with other areas before coming back to the main topic again. Here give a pause, read the question again & think again. Diagrams must be used effectively. Ending of the answers should always be visionary/positive and solution based with examples if needed or with some lines. There should be mixed use of paras and points. Examples should be quoted wherever possible but refrain from using one enterprise again & again.
For short answers, write the basic definition and then directly hit the core. Once again ending should be solution based. For merit/demerit/feature use diagram or points.