I usually look at my factories' needs and make sure my sphere includes the resources necessary. I also focus on goods that are 'top 3 imports'.
I always find my capitalists like to build luxury clothes factories, and therefore I want silk in my sphere. Unfortunately it can be quite difficult to get Japan, Guangxi, Chinese Empire, or Yunnan before the other great powers get in on it (especially if you start as a non-great power).
I find Korea is a good sphereling for many games because they coal, iron, and timber. These 3 resources can become scarce in the mid-game, as everybody gets sphered but before technology increases production to overflow the sphere.
Beyond the above tips, I focus on regional and strategic goals i.e. arranging for allies in case of war.