do it in 1 hit,
put yourself under exam conditions,
in the real hsc, they wont give you 1 hour intervals so you can take a break, have a coffee, go to the toilet, etc.
set aside 3 hours,
start at X hrs, and finish at X+3 hours.
the only way to prepare yourself is to put yourself under the same exam conditions including time etc,
if you're having problems though, just take your textbook out and revise, or take a peek at the solutions (if any) -- the first few lines to get your brain into gear, at least you'll remember it.