Practice is essential to develop writing skills. To build up concentration, start meditation. It will not only help you get relief from the exam stress but will also help you stay more focused on the topic.
As far as writing skills are concerned, practicing old exam papers and test series is the best option. Institutes like Vajiram and Ravi provide well-researched test series to help students work on their exam taking skills.
While taking mock exams count your time and try to complete every answer in pre-decided time period. The clock running in your mind won't let you divert from the topic and will help you complete the answer quickly.
You can also take help of your teachers at the IAS exam preparation coaching center. They are experts and know how to handle different situations. They will definitely tell you techniques to write answers with more focused approach.