Hmm, even though I am a notourious stress-head, I've found that if you plan well for each exam in advance (eg: writing out a quote/technique/effect/link etc table for each text you do in english), you dont stress so hard.
It also helps to have at least 2 plans of action for post-HSC life... If I get a UAI of 80+, I'll take a year off, save, travel, then go to uni and do a bachelor of secondary education and a bachelor of the arts or something similar... If I don't meet the UAI, I'll go to TAFE, get a Cert IV in IT then try and apply for a course in software and design in uni, depending on what's available and stuff.
It's really reassuring to have those things behind you... and if all else fails, deep breathing (NOT hyperventilation!) while thinking 'Only a few weeks to freedom...' really does help the stress relief.