1) Do not enter a patch of grass that is barely shaking. It is a decoy patch and doesn't contain a Pokémon.
2) Once your chain is started, DO NOT step in the patch of grass directly above you unless you clearly see a sparkle come from it. It is hidden by your head and difficult to see whether or not it was shaking.
3) Once your chain is started, DO NOT equip your skates or bicycle. This breaks your chain.
4) Once your chain is started, DO NOT walk so far that all the shaking patches go offscreen. This breaks your chain.
5) Once your chain is started, always try to step in a patch of grass that shakes in the same way as the one you previously stepped in. If you step in a patch that shakes in a different way and it does not break your chain, start stepping in patches that shake in that new way, not the previous way.
6) Once your chain is started, DO NOT run away from a battle. This breaks your chain. Capture or faint the Pokémon.
Hope I helped!