How about a subtle hint when they first start that you know a lot of people in the industry--we meet for coffee--and the other guys often tell you about people that didn't pay. "None of the good companies will touch those accounts."
Hint also that if a lawn is unmowed...most people...know, it is because the homeowner has not paid.
One times--give them 2 weeks. No reason to keep mowing when not being paid.
Its best to bill in the middle of the mowing month. And if they don't pay in 30 days...send a new bill every two weeks. And call every two weeks. Email also.