I just watched a lotta old cartoons, MGM, WB, Fleischer, Disney, 30s-50s stuff, as well as The Famous Artists Cartoon Course. As far non-cartoon stuff, Loomis helped. No really. As far anatomy or certain subjects I might struggle with, I have an extremely easy process.
1) Draw a body part/object/subject that I think I know how to draw. If I'm sure I don't know how, I do the best I can. NO references of any kind.
2) Look up references of the real thing. Take notes on all the details I got wrong or missed, come up with a process to break down how to draw the real thing.
3) Make corrections to my original drawing or make a new one if I'm way off.
4) Wake up the next morning, the very first thing I do is quiz myself and draw the same subject with no references and then compare it with refs to see what I learned. Repeat until I can grasp it. Repetition helps.