The most important thing is to be aware of how best you, as an individual, learn. Do you learn better from traditional environments? Books or tutorials? Person to person interaction? Trial and error?
For moden motion design specifically, I’d seek out others in the same line of work in your area. Talk with them, have a business lunch, or see if you can take a look at the work they are actually doing.
For AE in general there are several large skillsets that I have (more or less) developed which have served me well.
Existing techniques: What people are currently using. This gives you a good foundation and ability to do the most common things. More importantly existing techniques have been fleshed out and can vastly improve your efficiency
Overall project workflow: Knowing exactly how your part fits in with the overall project is lifesaving. Including learning the other softwares that surround AE and how AE interacts with those.
Reverse engineering: Seeing how others have performed something and then working backwards can lead to learning amazing new things as well as developing variations on that work. Its also very good to be able to reverse engineer your own works for efficiency as well as pushing the limits of what you can do.
Creativity: Its hard to teach creativity and the creativity we use is often very different than what most people think. Our final product is certainly intended to be seen, but the individual steps and methods we use in AE (or other softwares) are designed to not be seen. Our creativity lies in how we get from concept to final product (the layers/keyframes/expressions/effects/etc that we use). The audience is not aware of what we do, but that doesn’t reduce the importance of our creativity.