This is pretty much a continuation of what 1Time4thePpl mentioned.
Best way to achieve your goal:
1. Like 1Time4thePpl mentioned, convince yourself the UAI cut-off will be higher (which it probably will anyway). A great way to achieve the result you want is to stick post-it notes in your room, on the toilet door, or anywhere that you know you'll see it of the UAI you want to achieve. I did this and it worked quite effectively.
2. The second tactic is to write down 22 times for 11 days on a piece of paper your goal. But instead of putting it in future tense, put it in present tense. For example "I am getting xx.xx for my UAI". For each of the times you write that, put a comment underneath of what you are thinking. For example:
"I am getting xx.xx for my UAI"
"I hope I do"
"I am getting xx.xx for my UAI"
"I know I can"
etc
The reason for the comments is to show your change in state of mind on your goal. You may initially believe you wont reach your goal, but after the 11 days, you may be determined and motivated to reach it.
This is another way that helped me achieve my goal (although writing for 11 days straight is a pain. You can't miss out on it one day. You have to do it 11 days in a row).
3. Don't burn yourself out by over-studying. You need to have a social life as well. This will release the stress and keep your mind off the HSC. You can also get a part time job (but don't work more than 10 hours each week). I found working quite enjoyable during the HSC. Besides the crappy customers, work was great and it released the stress (by yelling at the customers
).
I hope that helps as well.