What I found was to first find what the average overclock of the card is. If you want to keep it on stock volts (which I recommend) then have a look at this thread. It'll help you know what limits your card can reach.
The next thing to do is just bump up your core clock up by 5 or 10mhz at a time and after every bump make sure you stress test the card.
You can do this with playing a game, Furmark or MSI Kombustor (Which I believe is bundled with MSI Afterburner (A great GPU Utility!))
During stress tests, have a look at the GPU Temperature and also if you see any problems with the display, usually you can see blocks in the picture that shouldn't be there. If you do see any artifacts then you need to drop your core clock until the display is back to normal. You'll then be able to find your max overclock for your card.