Overclocking in general is a black art and is nothing is really ever guaranteed to work when you tweak on the hardware level. The point is that every processor of any kind will perform differently when overclocked because no piece of silicon is the same. So your GPU may have limits that are very different from GPUs of the same type.
The other thing is, current GPU overclocking is very different from GPU overclocking several years ago. Nowadays, especially with the nvidia boost edition cards, the card will automatically overclock itself based on user defined parameters. It used to be that one would hand tune the clock frequencies as well as the voltages on the gpu core and memory.
To overclock, you set a power target, temperature target, and maximum clock frequency target. The card will attempt to push itself to those limits while still maintaining a stable state. Once the card deems it unsafe to go higher, it will stop boosting clocks. So simply adjust those sliders to your satisfaction until you achieve the reasonable FPS increase you are looking for.
The problem with GPU overclocking is that you generally can't change the cooling method, besides having a custom water loop installed. This immediately places a restriction on how far you can overclock before you even start. A side intake fan will help if your case supports it.
If you're running on the GPUs stock fan(s) then the general advice is to not overclock above 20% of the current clock speed. For example, a GPU clocked at 1000Mhz shouldn't be overclocked beyond 1200Mhz. The same applies to the GPU memory speed. The important part is to keep the temperature as low as possible; heat is the killer of PC components.