Reference from real life whenever possible.
When you reference or copy from another artist's studies/sketches/art, you may be copying their mistakes as well. Furthermore, an artist's studies are their notes and --just like with history or chemistry notes-- copying someone else's notes will not help you fully understand the material. To completely understand anatomy, you must take your own notes and build your own understanding through observation.
This is probably a no-brainer for many artists but... scruffynerfherder (https://www.deviantart.com/scruffynerfherder) and I were talking about this last night, and with the increase of "anatomy studies" showing up on dA's front page, it'