You have to raise the frequency in small increments for two reasons.
Each time the frequencies are raised, you want to ensure stability by running a visual demanding application, such as 3DMark (as mentioned above). You want to do this to catch any artifacts that may occur.
The second reason you want to raise the frequencies in small increments, is to prevent heat stress. You want the hardware (specifically the GPU), to adapt to the amount of heat that it will be subjected to.
Remember, safe overclocking takes time and patience.