I posted this on another forum, but I'll re-post it here. This is my method. I don't always get the highest clocks, but they are always rock stable.
Using ATI Tool, Kombustor, or EVGA's OC checker by itself isn't enough to find stable clocks with any certainty IMO. My GTX 470 will run higher clocks in the OC checker that it won't sustain in real world scenarios.
What I usually do is use something like ATI tool to determine what clocks the card won't do. Basically, ramp up the clocks relatively quickly (say 25MHz every 30 secs) until I get errors. Once I get errors, I back off the clocks a bit - say 50MHz. I do this for both the gpu and memory independently - meaning when I work on RAM clocks the gpu is at stock and vice versa.
Once I have some baseline gpu/RAM clocks I start running something much more intensive, say the Heaven benchmark maxed out/full screen, and slowly work the clocks back up in 10MHz increments every 5-10mins until it gives me the slightest artifact or lockup. Once, I get an error, I back it down at least 10MHz and then I bench intensely at these clocks for about an hour. If I get any sort of error, I back down another 10MHz and start over.
Heaven is good for this, or grab the Crysis Warhead benchmark tool (if you have Warhead, of course) and setup up each bench in say a 5x loop. If your clocks are stable at running Warhead or Heaven full screen at max settings for an hour, your OC is most likely stable.