How to maximize your chain
Let's start with what Lucas/Dawn tells you about the Poke Radar. When you see him/her standing outside of Professor Rowan's lab in Sandgem Town, he/she will tell you a few interesting tidbits about the Poke Radar:
"Hey! (your name), you've got yourself a Poke Radar, too? Watch for tall grass to rustle wildly when you use that Poke Radar. That's a sign that a rare Pokemon may be hiding there."
This refers to the patches of grass that flash slightly and usually indicate that rarer, non-Sinnoh Pokemon is in that patch, as we mentioned earlier.
"Sometimes a patch of grass sparkles when the Poke Radar is used. People say a different-colored Pokemon is hiding there! I wonder if that's true?"
This is a direct reference to the Shiny patch of grass! Besides our experiences catching Shinies using the Poke Radar, this is the best direct evidence that the Poke Radar is intended as a tool to catch Shiny Pokemon.
"Hi, (your name)! How's your Poke Radar working out? When you use it, do you sometimes see more than one patch of tall grass rustling? The patch farther away gives you a better chance of seeing the same kind of Pokemon in a row. I think that's how it goes."
This is the most useful piece of information that Dawn/Lucas gives you. ALWAYS choose the patch of grass that appears farthest from you when building your chain. But there are many more rules you need to know to successfully build a large chain...
Above: Dawn's/Lucas's tips provide a good starting point, but you'll need more help than that to have any chance of catching a Shiny