Use rivatuner, raise the core clk (originally in 600 MHz) in small steps and use atitool to tax the GPU and check for stabillity and temps, if needed increase the fan RPM, stop when you get artifacts in atitool or when the temp gets to 88 ºC and thake a step back.
Then raise the VRAM speed (originally in 900 MHz) in small steps and repeat the procces above, usually toy can get somewhere arround 650-700 MHz core and 950 MHz RAM.
The shader clk gets raised along with the core clk, both rivatuner and atitool are simple to use, try google if you need instructions to use the software.