well one thing i did a while ago was get a picture of a person, put it on one layer of a drawing program (could be any program, i used ibis paint), lower the opacity, and then just try and see what shapes were inside by making a new layer and drawing over top. it really helped me with proportions!
also just look at the way other people draw people. and try drawing them a lot yourself. i know "practice" is the worst piece of advice to give, but the more you do something, the better you get at it!