Tutorials, videos, books, and even a life drawing class never did anything for me when I was trying to get better at anatomy. It was like I couldn't retain anything I hadn't already learned first hand, and information like 'this is how you draw this' simply wouldn't stick without some kind of experience of my own for it to cling on to. So I came to know anatomy in a few other ways:
1. Study of biology/the animal kingdom in general.
A basic knowledge of how bodies are put together from the inside out is essential to understanding the limits of movement. The way arms hang at the side, the way spines curve inward towards the base, the fashion in which women's hips rock as they walk, the way men's don't--- you need to know what's going on underneath before you can take a guess at how things look on the surface. Natural gait and stance are both down to this.
2. Pay attention to people around you.
Play a game while you're out shopping and later throw up over how judgemental you are by watching people from afar and making assumptions about them from what you can see. Angry gesticulation, cowardly behaviour, childish behaviour, fear, happiness and dislike are all there for you to witness, and personally I think this kind of body language is essential to good anatomy. No point being Michelangelo if all you can draw is an upright body looking pretty
3. Be generally and genuinely obsessed.
Sometimes I just stop and look at something my hands are doing like 'how do I distill this in a sketch?'. I am also very rude and do it in conversations with people, especially ones with loud body language. I watch other people's kids and how they play, I watch people on the street and in their cars, I watch my family and try to understand what makes us similar and different. I can't help it, but I'm sure it helps. A genuine curiosity and interest is always essential to improvement, so try and figure out what you like about having killer anatomy skills before you force yourself to study. You're supposed to be having fun, after all.
End note: There is a period of time in which you will be rubbish at drawing anatomy. It is unavoidable and everyone goes through it-- just like there was a period of time where were were all bad at colouring inside the lines, tying our shoe laces and brushing our teeth. The only reason we're good at those things now is because we did them every day, even if we didn't want to, even if we hated it. The more effort you put into learning something and the more often you do it, the sooner you'll be out of the rubbish period and into the great period where everything comes naturally. Sooner or later you won't have to think about anatomy, you'll just know how to do these things, but remember you cannot avoid the period in which everything you do is naff-- you can only work through it.