We had the chief VCAA assessor for context come to talk us at school, and he said to succeed in this area (Context), you need 3 things:
1) Quality of ideas - (i.e. conceptualizing the main ideas from the texts and using them as a basis for your writing)
2) Linkage to the prompt - this is a response to the prompt, so there has to be a connection to what's being said (think of it as a springboard for your ideas). This basically assures that you don't show up to the exam with a memorized essay, or write whatever you like
3) Quality of writing - succinct, terse and sophisticated language. It doesn't mean using big words or trying to impress, it means using the right words. (if you do wish to experiment with verbosity, I would try and convince you otherwise by bringing in an anecdote, but it may just be easier for me to link you to this thread:
http://vcenotes.com/forum/index.php/topic,28594.0.html). As a smart playwright by the name of Shakespeare once said: "brevity is the soul of wit".
So basically understand the main ideas behind your area of study, link it to the prompt, and write clearly and effectively. Also, if you have trouble thinking on the spot, it may be a good idea to brainstorm a list of ideas relating to your topic and saving them in a notebook. Every time you write a context piece, just refer back to your main ideas. Good luck