First of all, use repels, not a cleanse tag. If you find a wild one, even if it's the same one you're chaining, outside of a patch, your chain breaks.
Pick a nature you want and breed a kadabra with it, so your shiny has a 50% chance of being that nature if your kadabra is first in the party. (you don't need to keep it there the whole time, just when you walk into the shiny patch)
Your chain can and will break if you go to a patch on the edge of the grass. If you do, the location chosen to shake has a chance of being outside the grass, and none may shake at all, breaking your chain. (you can tell because the music reverts to the non-pokeradar music)
And finally, the bigger the patch, the better, so that you can see more patches 4 spaces away.
Good luck, of course! =D