I end up finding things like this useful. For example, I have more experience with cigarette smokers than cigar or pipe smokers, so if I have a character doing the latter two things, I don't have a very big mental library for what that looks like. http://www.artofmanliness.com/2015/02/24/how-to-smoke-a-cigar/
I love this fucking website: http://www.artofmanliness.com/2015/02/25/suit-sport-jacket-blazer-difference/
Has anyone posted these? http://humanae.tumblr.com/random Paste this repeatedly to get a random face, it helps with learning about variations in faces and skin tones. https://sketchfab.com/ - 3d models for reference
http://www.quickposes.com/pages/timed Like pixelovely's gesture drawing, but this also allows you to zero in on certain features like the face and hands.
tracyjb of Lackadaisy put out a journal on character design: http://tracyjb.deviantart.com/journal/Lackadaisy-Patreon-416956624