it is possible to improve, but not necessarily ace them..
like, a friend of mine added around 20 marks on top of his trials, but he said he just became a hermit, didnt go out AT all in between the trials or hsc, and just studied ALL the time.
popular belief is that 10 marks can be added on top of trials score, if you are prepared to study..but, it all depends on how much work you are willing to do.
oh, and it also depends on how much your trials are worth at your school...like, if their 50% your go-ooone (lol, i luv tha footy show), coz ur rank will be screwed, but if their only like 15-20% its heaps better