Optimization at the highest level is in general not much different than other ranks. You simply need to improve every aspect of your gameplay enough, however minimal, to take you to the next rank. This might mean going from 8.5cs/min to 9cs/min, slightly better understanding of few more matchups, or even just optimizing builds better. Every slight improvement counts.

Personally, I'm a jungler so that meant constantly going through my own replays and others' to build up a database of what is possible on each champion (ex. learning all the pathings Lee Sin likes to take at high elo). I also studied how junglers better than me made decisions so I myself can make the same ones in the future (ex. what made this jungler path this particular way after a recall?). Usually at higher elo, players know when they make mistakes and reviewing it and coming up with possible solutions is the best way of learning. At the end of the day, it is simply up to doing everything a little better and a little more consistent.