@Ace (sorry, but you're name's too long :P) - It's even more essential then that you read more Chinese stuff to expand your vocab range. It's a big step between Chinese GCSE and Chinese AS, you need to know stuff about the modern day society and you have to be able to translate Chinese into English, which is usually thought of as easier than English into Chinese, and you have to write an essay, you probably know all this already.
You can improve your vocab is by reading, since they don't have a vocab list for AS Chinese, unfortunately. The easiest is probably to have a nice stack of lined paper and a dictionary at hand while reading news online. It's boring, but you'll be surprised at what you learn. If you take Chinese lessons, it might be a good idea to ask your teacher to recommend books and dictionaries.
The easier, and possibly faster way of improving vocab is having conversations with other people in Chinese at least once a day. I did that and it helped lots, but then again, I have lots of contact with Chinese speaking people.
Good luck with it ^^ just try your best. I hope I've helped ><.
Edit: I forgot, there is one other way of improving vocab, but it involves getting an English-Chinese Dictionary, which I have had trouble finding in England, but it might be because of where I am... Anyway, you can try writing a diary in chinese. That would really help. But it can be very time-consuming... And during the day, whenever you're saying or thinking or writing something in English, just let yourself wonder at how you would say/write it in Chinese.