The amount you can overclock varies from card to card. Start by bumping up the core clock and the memory clock by 5-10MHz at a time. Play a game and watch your temperatures. You'll want them to stay below 75 oC at least, I'd personally cut it off at 70 oC. If the temperatures are okay after several minutes of gaming (play the most demanding game you have), then bump it up a bit more. If it crashes, either roll back the clocks or bump the voltage SLIGHTLY. Doing a voltage bump will influence your temperatures the most. Keep trying small bumps and testing until you find the sweet spot of voltage, temperatures, and clock speed.
Instead of a game you could also run a benchmark like Valley.