Ok before we get started just to point out i am not responsible for any damage to your GPU or PC you do this at your own risk.

Ive Moved this from PC Building

Pros
Better Performance
Higher FPS
Smoother Gameplay
Less Framedrops

Cons
Can Shorten the lifespan
Can break the card(if your silly about it)
HEAT
in somecases can void warranty

How do i successfully overclock?
Well you going need to need some sort of overclocking software, there free so don't worry you dont have to pay for them
EVGA Precision X
MSI Afterburner
Asus GPU tweak

For this tutorial im using EVGA Precision X
Your also going to need some sort of GPU benchmark.

Unigine Valley
3D Mark

ok so load up your overclocking software
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you will notice mine my fan speed is at 0 just ignore thats its cause i dont have fans for my GPU as they have waterblocks on them

Power target is the Watt so if my titans are under load the max wattage they will go to is 250W because thats what they are stock.

You can increase that to allow increase overclocking potential and stability(in somecases).
on air i would increase to around 105% (i think the limit is 106% on a stock Bios)

GPU Temp Target is when the GPU gets to that temperature it will start to throttle, this is a good thing as your card wont overheat die but you will lost performance also experience frame rate drops or skips. Personally i would leave this as a fail safe should your card get hot.

GPU Clock Offset now this is Main part. This is the Brain of the GPU this is what effects the performance of your card. On air i would recommend putting a 100mhz+ and then test and tweak around there.

Mem Clock Offset is pretty much the same this is what controls GPU loading and Pre buffering increasing this will make games load objects quicker textures and so on. you could probably put a 200Mhz+ and then test and tweak around there.

Now Overvoltage is an amazing help but also is big deal, increasing this can increase the overclock potention pushing the clocks higher but also generates alot of heat and if you increase too much can burn out the card. i would only recommend doing this if you have a watercooled GPU or a decent air cooler with good airflow. but a nice voltage to keep stable would be around 24mV increase.

Now that you have overclocked you need to test stability
so open your benchmarking software and make sure its quite high settings also turn v-sync of as your benchmarking so you wwant the GPU to push its self.
Keep and on the Temps if there staying low and your stable increase a little more.

i would recommend increasing by 10Mhz-/+ at a time so you dont over do it.
the voltage i wouldn't mess with too much unless your a watercooler.

Heres what i do with my Titan X when it was air-cooled
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This here is what mine is set too whilst watercooled with a custom BIOS and with Extreme overclocks.
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Theres not much i can show you as it depends completely on your card how you do though if you need help please message me and i can talk you through it.