A common question asked by a lot of students at all levels!
You must remember to always refer back to the mark scheme for the exam that you will be taking. When looking at past papers, always have a copy of the mark scheme attached to that exam to hand. When you are writing out practice answers for the first time, try looking at the mark scheme and include everything which the mark scheme says to. Remember! Examiners aren't that mean, they WANT TO AWARD YOU MARKS! Hold the examiner's hand, lead them through your answer and produce detailed yet simple, straight-forward, short and sweet answers. This is the best way to a top mark.
Most exam boards award marks based on A01, A02 and A03 criteria. Put simply, this is a step-by-step process that examiners use to work out how good they think your writing is! A01 assesses your knowledge and understanding of the subject based on the information you provide in your response. A02 tests how well you are able to use your knowledge to form a coherent argument. The best students will use a range of concepts and theories and constuct really tight arguments with clarity. Finally, A03 marks are awarded based on how fluent and generally well written your essays are. Good political vocabulary, grammar, direction and essay structure 'polish off' a good exam, but are also crucial to forming the best essays! How do you make sure you hit all three criteria? Structure your response! Give the examiner the exact point you want to make, give them examples and theories from the subject which support the point you are trying to make, explain, in full, why your point make sense and give a compelling and well written argument!