I think I did well in my speaking exam as I think I ruined my written exam and still came out on top. What I found really helpful was going to any and every japanese workshop day you can find. I went to the Kirrawee centre twice which focused on speaking and listening which I found very helpful - the entire two-day program was run (almost) entirely in Japanese with native speaker trainee japanese teachers which was great. I also went to a practive speaking day at SHORE grammar which was great. 8 practive HSC speaking exams in a row done with teachers, which we taped, and all their feedback.
Although it's obvious, practice speaking as much as you can. It doesn't even have to be with anyone else though! Practice saying sentences in Japanese all the time, waiting for the bus, walking home etc. It's important to get correct intenation and timing with sentences, not having to take a big breath near the end, getting tongue tied etc. Speaking with tone and emphasis is also important. Markers are specifically told to cut-off / interrupt candidates who launch into a learnt by rote speel about anything - they can often tell because the candidate has practiced saying it so many times it comes out like a robot. Keep it fresh, practice including something recent like a family birthday or trip out with friends if it's appropriate. This will show the marker that you can actually communicate information about recent (and most likely) relevant events.
When ever you think something or say something, think to yourself "could I say that in Japanese?" If so, practice it a few times. Talk to your teacher a lot because I found that in my class speaking was neglected for a lot of the year before we focued on it at the end. Try to organise a practice speaking exam - it's only max 10 minutes and get some detailed feedback.
Think about which grammar points you could use for certain types of questions. -tari -tari really doesn't go with more than one or two types of questions, everyone. Saying the words out aloud will also help your vocab. I've heard rumour that dictionaries might be scrapped in coming years. I found it best to learn *all* HSC vocab off by heart with flash-cards, saying them aloud. Using one very appropriate and exact word in a speaking exam is a lot clearer and more fluent than having to explain them.
ie. "I have to do both english and religion at school" could easily be "English and Religion are comulsary subjects" especially if you've already used nakerebanaranai.
Wooh, bit of a long rant. One of the biggest assests in the speaking exam though is *confidence*