How do I build up and retain a sufficient level of vocabulary?

The need to build up a sufficient level of vocabulary and retain it is extremely important when taking on the written and oral componants of the A level exams both when it comes to to comprehension and expression. For this reason, my advice would be to undertake short reading exercises on a regular basis during which you will come across new words and expressions. These exercises should be chosen around certain themes and when you are confronted with new vocabulary you should be sure to either enter it into a list of terms attached to this category or, to insert it into a mindmap that you have contructed around this theme.
This should enable you to learn and categorise a higher level of volcubuary which you should then aim to use during short writing exercises. These exercises consist of you setting yourself a theme around which you have built up a certain amount of new vocabulary and to writing about this theme, utilising as many of the new words and expressions learnt as possible.