Here's what I did to overclock my i5-2500K on a P8Z68-V PRO/GEN3:
Download CPU-Z (monitor speed and voltage), RealTemp (monitor core temps) and Prime95 (CPU stress test):http://www.techpowerup.com/realtemp/http://www.cpuid.com/softwares/cpu-z.htmlhttp://www.mersenne.org/freesoft/
Visit this page:http://www.overclock.net/t/1042186/asus-z68-series-information-thread-drivers-bioses-overclocking-reviews/100
Scroll down to the post by "SimpleTech" and follow those exact instructions but replace the Turbo Ratio with 43 and initial CPU voltage at 1.31V.
After tweaking in BIOS open all 3 programs and start the Prime 95 blend test.
Leave that on for 1-2 hours (longer if you want) and watch the temperatures - I'd worry if it hit 75+C. Make sure you turn off sleep mode in Windows.
Then, you basically repeat the process but each time, you reduce the voltage by 0.1V without changing the Turbo Ratio. Once you get a BSOD (I got mine within 15 mins of not having enough voltage), you revert to the last CPU voltage that Prime 95 ran stable for 1-2 hours.
Then, you can run Prime 95 for a longer time (24 hours is not surprising) to confirm stability.