As you probably know, CS is an exam geared to test whether you can elicit history from a patient in a decent manner.
I wont go through all the details of the exam as that's not my point. My point is to let you know two things:
1 - You should take the exam seriously, like any exam.
2 - At the same time you should not stress out about it because the real aim is to see whether you are a decent person that can take decent history. They are not out to fail you.
Now that means you will need to study seriously and prepare, but that you needn't worry if you forget a specific detail in a specific case. Based on my experience the people that made the exam are reasonable and they wont fail you if you forgot to ask about smoking habits or sexual orientation. They care about your overall performance: everything from your approach to the patient through to asking questions that are relevant to the main issue at hand.
With all honesty, I probably missed many little details during the exam and thought that was horrible, but I did address the main issues every time and of course I treated the patient with respect and proper etiquette. That is what is truly important.
With regards to resources my advice is to use either the first aid for CS or the USMLE WORLD for CS. The advantage to world is that they have a complete course online that also includes videos of the proper ways to examine a patient.
That is actually very important, especially if you're trained out of the US where there are different approaches to the physical exam (draping etc.). If you're really stressed, just use both resources. Of course, mock exams are key here as well and you should practice history taking and physical exams with a buddy before taking the test.
The amount of time necessary to study well is about two weeks, it can also be done in less time (I took nine days) but I believe two weeks is better.
I cant say too much with regards to test centers, I took my test in Atlanta and they were excellent in every respect.
Finally, an excellent website with regards to CS is
USMLE Clinical Skills Exam Tips
Application & Interviews