Don't fret! I can't always come up with ideas off the top of my head either, and I don't think many people can do it easily either!
I use countless sources of references, photos, tutorials...! Hell, I needed a reference for the shirt in this picture like you couldn't believe! I suck at clothes...!
My advice is to keep studying the human body, use as many references as it takes to get familiar with it! All artists, great to small have used other artists and still life as tools to teach them along the way! Hope that helps! <3