Art studies

'Practice makes perfect' applies to art as much as anything else! Repeatedly drawing something you struggle with is a sure way to improve, but studying photographs or others' work can also be a way to improve. Mastering lighting, line weight, anatomy, composition and more relies on observation and practice.
Do you conduct studies very often, or do you very rarely/never do them? If you do, is there a type of study you find yourself redoing often?
Which resources do you use for your studies? Share them if you have any recommendations for others to use!