Most people use Afterburner, yes.
As for general rules: I´d probably google a bit and see what a "safe overclock" for you card seems to be and try it out (maybe a little lower for starters). Then use a graphic intensive stress test like Furmark or Haven or whatever most people currently use. Check for graphic issues and monitor your temperatures.
How to push it further:
Increase your clock speed by 20MHz or so and stress test it again. Repeat this until you experience graphic issues or high temperatures.
When you reached a safe GPU overclock, try to overclock your memory. Usually, this won´t nearly go as high and usually won´t increase your performance that much either.
After that, you could theoretically play around with your voltages which I personally havn´t done in a very long time.
Also, always be aware of the risks that come with overclocking.