MSI afterburner, max out the volts and power limit...
Then I usually set the fans to max while stress testing, then up the mhz in increments of say 50mhz(?) then refine it when it becomes unstable.
Stress test with Unigine Heaven, and 3 loops is normally relatively accurate for stressing, although many would argue that gaming is the only way you can be totally certain. For example, my 670 is stable for 3 loops of unigine but if I leave it at that high an overclock it isn't stable on BF3.
You just have to play around with it really. Don't use Furmark or any furmark style program as they can wreck your GPU and try not to let temps go above 65-70ish.
People can't really tell you much more than that tbh as it'll be massively different depending on your setup.