Getting a hang of the technique is really important for the writing section. There's often key aspects that the examiners are always looking for (use of the subjunctive is one if you're doing AS). Best way is to do lots of past papers really but as far as learning goes, it's about vocab and learning complex structures that you can then put into any essay regardless of the exact topic, your textbook should have examples of these sorts of structures, usually in the grammar section. Stepping up from GCSE to A level you also need to remember that they will be much harsher on basic errors like adjective agreements.