hmm it's not something that you can't learn. look at artists - to learn how to draw well we all draw (npi) from established artists.
other than reading exemplar responses, which can be depressing, go over your own work and try and give it that extra oomph which will score you those few more marks.
vocabulary is something that everybody should aspire to improve; you don't have to use the longest word in the dictionary but using more complex words in correct contexts gives the marker an indication of your grasp of the language and that you can use it effectively. for example, instead of saying 'there are many kinds of change, including the change in the weather' you could say 'the seasons are but a single thread in the extensive web of changes' (or some such crap). THAT's artistry.
Don't be excessively verbose though (like my example above) - you just want to make a point, and to use nice words to say it.
Hmm. If you find yourself writing a lot of short sentences, see if you can concatenate them at all - sometimes one longer sentence is better than two. Conversely, if you write huge chunks of sentence ('on tuesday i did this and this and this and then we went here, before doing that at this place, where this happened two years ago') split them up and make them easier to read.
Try and make your argument flow convincingly. Even though each paragraph may be a new point, try to link them in some way ('However,...') or to make a logical progression through your essay (or whatever text you're writing). Don't talk about colour in one spot and then come back to it a few paragraphs later in more detail - it seems a bit staggered and not thought out properly. Most my replies in this forum are not thought out properly.
How to do that? Hmm. Practise writing essay structures - you don't really have to go into a lot of detail assuming you already know the texts inside out. If you can start making your points lead onto one another then you're on the way to learning the art of essay writing
Flair is some BOS-speak for "makes it look easy". You want your essays to just roll off the tip of your pen, or at least to seem that way. An experienced marker can tell if you've sat there and struggled to get a sentence out - it shows in the words you use and the copious amounts of crossing out on the page. You learn, over time, to make it look like you do this every day.
And by the HSC, you probably will be doing it every day.