I periodically take time off, go to a mall and document as many people as I can in my sketchbook. I recommend any foodcourt because you're bound to find diversity in face/body and age. A lot of my characters stem from remixing and combining these sketches of real people.
EDIT: And an additional benefit to these outings is that I can take the time to make quick gesture drawings that help me understand how certain people stand or hold themselves and their stature.