The key to maintaining the #1 ranking is a strong finish to the season.
Of course, in theory, if you want to keep your ranking consistent, then you need consistent performances throughout the whole year, but the season finale is especially important because it carries quite an amount of points and the WTF is also important for establishing yourself among your rivals and sending them a clear message for the next season.
Bottom line: you don't have to be at your best the whole year, but don't slack in the last two months of the season. There's quite a bit of points to grab here and it will surely be your leverage next year, even if someone is outperforming you a bit.
It's about prioritizing and min-maxing. A Nadal will maximize his points total during clay season so from that view he should absolutely play every clay tournament in that 7 week stretch. A Federer with his indooriness does the same with the year end schedule and capitalizes on all the worn out competition. And since the WTF has disproportionately large amount of points he's been able to hang around the top 3. But Djokovic is in such a rich vein of form slams-wise that he shouldn't need to min-max like the other players. Two slams plus his masters results should be good enough for #1. He was on fumes last year-end and still finished ahead by so many points and a three-slam/two-slam season is enough of a message.