1. Clarity of Purpose:

2. Motivation:

3. Dedicated Activity:

4. Commitment of Parents/Family:

5. Room for Relaxation:

6. Subject Selection:

7. Be Simple:

7. Current Affairs is the key:

1. First of all, everyone must understand that the Interview part (Viva + Psychological) is almost as important as the written portion, if not more. Whereas we all tend to work really hard for the written, spending almost a year preparing it, we take the interview very lightly. Some people even say, there is simply no need to prepare at all. I beg to differ with them for Interview does require preparation. The reason I say interview is very important is that all the candidates who clear the written examination have their scores lying in a narrow band of around 100 marks (between 610 and 710 marks). This means that at this stage 300 marks of interview gives you a good leverage to make the difference. A high score of above 200 could very well land you in top 20~30 positions even if your written score is just an average one along with 600 or so other candidates.
2. The second reason I emphasize on interview is that in my opinion it requires almost as much hard work as the written part. A dedicated preparation of 2~3 months, preferably even before or immediately after the announcement of written result would be my recommendation. Again, you must understand that by scoring 240 marks in interview, you can take a lead of almost 100 marks from a candidate who did just average in interview scoring around 150 or so.
3. The third most important thing to realize is that Psychological assessment is a very significant component of the whole interview part. In my personal opinion it is even more important than the viva voce. The reason I say this is that the Psychiatrists spend two complete days judging almost every single aspect of your personality in depth. The personality profile prepared by them is given a lot of weightage by the FPSC panel, increasing so in recent years. They sort of go along with the advice of the professionals, and only reconfirm their assessment through different questioning techniques. The variation between the psychologists and FPSC panel cannot be markedly large. So, give you best effort to prepare for this part so that the psychologists draw a good personality profile of you and recommend you as an officer material.
4. Prepare for group discussions and command tasks with your friends and other qualifiers. At this stage, you can also join an academy where you would have a chance to interact with many students at one place, something you cannot do on your own. Within the group discussions, adopt a stance which is moderate (not to extremist on either side). Your argument should be backed by logic and evidence. Your choice of words should be mild, but your tone should be firm and give an aura of seriousness. There is no need to shout and raise your voice unnecessarily merely to capture attention or drive your point. If your point is valid, it will be registered even with a soft voice. This way, an even better impact would be created that you are confident on your knowledge and argument and do not need the support of your oratory.
5. For the Command Task take time to properly explain the problem to your team members instead of rushing through at this point. If your team does not fully understand your problem, they would be unable to help you or contribute in your problem. A few seconds saved at this stage can in fact cost you later. Once the problem is explained to team members, ask them if they have completely understood it. If any member is unclear on any aspect of the problem, this would be a good time to clarify things. This would also give a positive impression that as a leader you take care of how your subordinates comprehend you. In my frank opinion, there is no need to pre-assign designations and appointments. While wrapping up the discussions, you can simply point to your team members and assign them tasks.
6. Try to think through different questions which might be asked on your personality both in the proforma as well as by the psychologist panel. Instead of these questions coming to you as a surprise, it is better that you have done some working on them. Questions like, your fondest childhood memory, things you like and dislike about your parents and friends, your domiciled district etc need to be prepared before hand. The forum would contain lots of questions asked by the psychologists to different batches.
7. For the viva voce, try to build the opinions on different issues. Try to ask yourself questions. Read one or two newspapers. If possible start reading TIME and NEWSWEEK. Again, on your own, work out small questions on any issue that can be asked from different angles. You should have answers to cover these angles. At this stage, also go through actual statutes, laws and provisions of constitution that are often referred to in news item and articles. For instance, if Section 295 (Blasphemy Laws) of CrPC is mentioned repeatedly in press and media, you should read and understand the actual provisions verbatim. One good online source for such documents is: The Constitution of Pakistan and Pakistan’s Legislative History

I hope it helps. You can ask me about specifics, which I might have missed out.

Again, it is a personal belief, but God is the One Who has helped me beyond my capabilities. I was able to do CSS and an MSc from LSE UK in parallel. I got such high marks in interview and passed the essay paper on edge. So, get close to Him!

Regards, and Best of Luck