Pretty much everything I could recommend has already been mentioned.
Personally I use either an example from a manga or I'll imitate the position if I can.
Another possibility is people watching. Find a quiet spot somewhere, select someone, and practice drawing them. But whatever you do, do not stare at them for long periods of time otherwise it creeps them out. Select someone on the other side of the room, take a quick glance, then look back at your drawing.