I'm doing the A2 exam on 14th June. We were always advised to do the essay question first then it's of the way. If you leave the essay to the end it might be rushed, and the essay is worth more marks than the other sections so if you're in a situation where you're not going to get everything done in the time allocated, it's always better to rush the reading/listening than the essay. Of course, not having to rush any of it would be ideal, so make sure you've done a past paper in timed exam conditions before the exam, then you have an idea of how you're going to divide your time between sections.
For the essay, learn some essay phrases to get your marks up for complexity of language, and try to use synonyms to get your range of language marks up- for example, don't just repeat "beaucoup"- if you're using it in the context of something happening often, say "souvent" or "frequemment", and if you're using it in the context of there being lots of something, say "plein de" or "un grand nombre de". Try to get at least one subjunctive phrase in there to make the essay more sophisticated. Don't spend too long on the introduction, just briefly say what you're talking about and then get straight in there with your main body. Keep referring back to the title and don't use a lot of information that's not directly relevant to the title. It might help to quickly plan your ideas first to give your essay a structured feel.
You may or may not want to learn whole essays, but make sure you can come up with some ideas for things to say on each topic- maybe some for and against points or something. Remember that you'll have a choice of 3 essays (if you're doing AS) so hopefully one of them is going to suit you.
Check your essay for mistakes. Sometimes while I'm writing essays, I'll make some mistakes on quite simple grammar points, and when I look back at the essay I'll immediately know what I've done wrong. When I've finished my essay, my head is so full of essay that it can be hard to see where I've made mistakes, so do another section before proof reading your essay and the mistakes should jump out at you more easily that way.
I hope some of these tips help. Try not to worry about the exam too much and good luck!