There is a book called SOLO that should should definitely read. Check your local Library, chances are, they have it. What it is is a book that outlines all the stages needed to create a HSC Drama piece in the allocated time given.
Other than that, my general tips would be (with evidence from my own experiences):
Read the posts in the BoS Drama forum. All lot of them are really useful on the do's and do not's of HSC Drama.
Play to your strengths and interests. I was interested in mime and silent theatre, so majoity of my piece was non-speaking action and mime. If you like comedy, do comedy. Likewise with drama.
Write a basic outline of what you want to achieve with your piece. Mine for instance, was essentially : To create a silent piece, that portrays an important theme of life. What that theme was or the setting was irrelevant. The point is, it provided me with a foundation to get started.
Draw inspiration from everywhere. TV shows, movies, radio, plays, musicals, newspapers, journals, stories, songs, poems, articles etc. FInd a topic that seems interesting, then mindmap possible ideas. Mine was inspired by a segment on The Sideshow TV show, but also Waiting For Godot, Mr. Bean, and a plethora of other sources.
Make sure you write everything down. Carry a notebook with you, or in your bag, from now until the actually performance. A phone with 'notes' settings can work as well, but your less likely to write big sections of ideas. By writing down everything, you have more ideas to play with.
Record your logbook as you go. Seriously, it makes it so much easier than forging one at the end. Also, you get to see the progression of your piece, and can refer back to ideas that you originally had dismissed. It really is worth it.
FINALLY, pick something that is important to you. If your believe the human race has become shallow, create about that. If you think technology has overrun our lives, write about that. Animal rights, death penalty, whatever you feel most strongly about. if you run out of ideas, cliched topics can work (that's why they're cliche) but find a way to do it differently. I can guarantee markers haven't seen many Kabuki style abortion pieces. Do what you want, and express what you means the most to you. That's my biggest tip.
Goodluck. It's a difficult process, but it's most certainly worth it in the end.