Here you see my gtx 260 at full stock clocks. Time to crank it up. The first thing you will want to do is turn the fan up and see what is a comfortable volume to you. I sit near a noisy air conditioner so the noise added is unnoticeable so I will crank it up to 90, but choose what you think is acceptable
Then you will want to bump up the base clock small bits at a time. maybe 25 or so. The graphics card will only adjust in set increments so if you do not see the clock change after you have moved it a small amount then possibly try moving the number just a little higher to make it register. after you have bumped the clock up you should try stress testing it with occt or any other gpu stress tester of your preference.
Repeat this until you get a crashed driver or artifacting or a restart. at that point you should throttle down the overclock a little and then move on to the shader clock and the memory clock. Repeat the same process and see how far you can get take note of the difference in max temp from the stock clocks and the oc'ed card to see if it generating a lot more heat without getting you much benefit.
Once you have repeated the oc'ing process on all three sliders you should try going into games and making sure everything is stable. if you get anything weird going on just try scooting them down a little bit.
Here is how far I got:
Best of luck. make sure your thermals aren't too high. 85c is about as high as you would ever want to go. If you exceed this then you will want to get some other sort of cooling like a water block or non-reference blower