Chaining is difficult indeed, but that's what make shinies even more valuable. Be prepared to see your chain break without any reason nor warning.
A chain might break if:
- You encounter another Pokemon.
- You encounter a wild Pokemon trough a random encounter.
- You flee from a battle (you must beat / catch the wanted Pokemon every time)
- You go outside of the grass, on the edge of the grass or too far away from where you started.
There are however a few tips to prevent that from happening (most of the time):
Go on the one that is shaking the loudest every time. Go on the patch of grass which shake the same way than the first one. It has higher chance of containing the Pokemon you are chaining.
Go on patch of grass that are 4 cases away from your character. The nearer, the higher the chance to break.
If you're unsure, wait for the next one. This is especially true once you score higher than 20. 50 steps is not a big deal.
Buy Super Repel (they are the most cost-effective). On you start your chain, be sure to always have one activated as a random encounter will break your chain. I always have over 200 on me.
Don't go on the patch of grass next to you, it has higher chance of breaking your chain.
Don't go on the patch of grass behind your chracter. It's a tricky one but the patch of grass might have been shaking without you noticing and it might have been
Remember your goal. Once you're at 40, just wait for your shiny grass without going on another one, except if you like your life risky (but at 40 Skitty, I doubt you will). The chance of encountering a shiny won't really increase and you don't want to break your 40 and start all over again.
Remember which patch of grass were shaking if you take breaks. As your chain grow higher, you might want to take break into the real world. You can but don't turn off / save the game and make sure you remember which patch of grass were shaking so you don't step into them.
A Skitty is harder to chain because if you encounter a "random Pokemon" in your patch of grass because the game is unfair, it has higher chance to be a common Pokemon, thus it breaks.
You might be interested by this article: http://kotaku.com/wanna-catch-your-pokemon-in-a-shiny-new-color-in-x-y-1450801224