i always write on a huge calendar what yards i cut on what day. i put boxes by each name to check off. one box to check off when the work is done and another to check off after they pay. it's easy to keep up with that way.
i'm solo though and the most yards i would cut in one day is 5. our yards are big around here. it's not much to keep up with to be honest. i've never had to go chasing down my money or anything.