First, install Prime95 25.5 and CoreTemp (0.96 is prefferred, but hard to find now).
Then, in the BIOS, disable SpeedStep and C1E. Leave TM2 enabled.
Set your memory to reasonable timings, like 5-5-5-15, with a memory multiplier ratio of 1:1 (2.0 for gigabyte boards).
Increase the FSB by 10Mhz each time, and test for 10 minutes using Prime95, and watch the temps using CoreTemp.
Continue increasing the FSB until you get errors in Prime95 (or reboots, or BSODs possible too).
Once that happens, you need to increase Vcore. Test again, and increase vcore until Prime95 shows stability again. Then increase FSB.
Do this until temps get unreasonable, or vcore gets too high.
I recommend 70C and 1.45v as maximums.
Once you reach your max stable OC, then test Prime95 small FFTs for 24 hours.
If you want to, then now is the time to tweak your memory speeds and timings.
Test them with a bootable Memtest86+ CD, running for 24 hours.
When you're all done, you should also crank your FSB DOWN a little bit, to give a bit of headroom for 24/7 running.