When i negotiate with teh homeowner i just tell them here is my WEEKLY price you pay every 2 weeks or once a month whatever is better for you. i set up a start date and a end date.
Then have them sign a service agreement and it states that they pay XX amount every 2 weeks/monthly whatever we agreed to. If they call and dont want the grass cut they still have to pay the same amount.
All my customers just pay when they are supposed to and never ask me to not cut even if im barely taking any grass off the top.
Ive had problem people in the past i just dropped them. This is part time for me so having or not having people is not a big loss to me at all.
I make most of my money in hardscape work.