There is a special formula when using the PokeRadar that says when you encounter the same Pokemon over and over, the probability of encountering a shiny increases.
This is called "chaining". You also need to use Repel so you don't walk into any other Pokemon on the way to the rustling grass.
The longer the chain, the higher the chance of encountering a shiny, up to a chain of 40 after which it's about 1/200 each time. It's VERY difficult to get long chains though!