Although I'm currently doing French and Spanish AS (and getting As) this might not be as helpful as someone doing A2. I'd say, to begin with, rule out your weaknesses, whether it be reading, listening, writing, grammar or speaking, and concentrate on those.
If you're doing Edexcel, like me, there are certain questions which are harder than others - namely Q4 and Q7, which are overly pedantic and harsh in the mark scheme. So, it might be an idea to concentrate on certain types of questions that you find difficult. And do past papers in general, too.
I personally haven't done the speaking exam yet (mine's on the 7th
) but what really helps is practicing. I'm not sure about your situation, but in my sixth form, we have speaking lessons every week, which are essentially a mock exam with a previously used stimulus. If you don't have that, don't worry about looking weird talking to yourself in your room. Just speaking makes it feel so much more natural when it comes to the real thing, by making your mouth get used to making the different shapes and sounds.
If your writing is a concern, and your teacher doesn't mind, try writing essays for them and ask them to go through them with you, or posting them in the forums asking people to make/explain corrections.
Also, learning certain filler phrases such as 'que yo sepa', 'pues', 'bueno', 'de hecho' and 'o sea', makes it feel more natural and less awkward if you have a pause/need a few seconds to think of an idea. And another thing - if you go completely blank, ask your teacher/the examiner to repeat the question even if you understood it, to buy yourself some time!
Although I usually wouldn't recommend that you learn vocabulary in the same way that you learn facts, you might want to do that. It doesn't really work for me, but if your vocabulary is an issue, learn some of the basic words they use in your unit. It might also be an idea to learn some fancy essay phrases to bump up your marks for writing. Again, if you're doing Edexcel (not sure about the others), sometimes they make you write a letter. Even if you write like a native but don't use the correct format (formal/informal letter, article etc), you can instantly lose a grade. So definitely learn how to greet/sign off a letter. If you like learning vocabulary sort of how you learn facts, by repeating it over and over, try sites like memrise.com (free.. and it's good for other subjects) vocabexpress.com (extremely annoying but helpful I guess. Might need an account through the school) and quizlet.com (again, might need an account through the school)
As for reading and listening, all that can really help is... doing it. The more you are exposed to the language (and take it in), the more you understand. It helps with vocabulary too. With the Edexcel textbooks at least, we're given CDs with listening exercises on them. They are a bit easier, from my experience, than the actual exam and textbook exercises, but it's better than nothing. If you don't have that, and prefer doing the past paper listening questions, do those.
I'd highly recommend that you identify certain tenses or parts of the grammar which you don't understand, so you can either look it up on the internet for an explanation, ask your teacher, or ask for help in the forums.
This is so much longer than I intended it to be, but basically, identify what you're not so good at... go over it... then practice, and practice some more! I can't really stress enough how practicing and learning from your mistakes are the most important things when it comes to Spanish, as it's not really something that can be crammed the night before.
¡Suerte!