There are various ways you can prepare for the ACER exam.
Practice the tests beforehand, simulating test conditions
Think about the ability areas that you feel are your weaker areas. Make sure you read around the subject and refresh your memory of any principles or concepts you will need to use in the tests
Get a good night's sleep the night before the test, so you are refreshed and have a clear head
Ensure you have eaten before taking the test
Stay well hydrated before the test