One of the best ways to improve your skills in any creative field is to study the work of others. Find some killer piece of animation, and then go over it with a fine-tooth comb. Tear it apart, find out what makes it work. Try and decipher what techniques were used to create it.
With that in mind, I want to share a Youtube channel today called Onion Skin. On his channel, which is dedicated to the topic of animation, Colin has a series of videos called "Read Between the Frames" in which he examines animated sequences on a frame-by-frame basis, and figures out how they were put together. My favorite example of this (from my favorite animated show Rick & Morty) is embedded below. Check it out, and then check out the rest of what Onion Skin has to offer!