Vent: A single late invoice payment nearly sank my whole month

Last month, I finished a big fence job for a new condo complex, a $4,500 invoice. The property manager said payment was 'in the system.' Three weeks went by, nothing. I had already paid my two helpers and my material supplier, so my own account was almost empty. I had to call them every other day for a week, which meant I wasn't out looking for the next job. The payment finally came through on the 29th, the day before my rent was due. It made me see that my 'cash flow' was really just one thin line, not a buffer. How do you guys plan for when a client just... doesn't pay on time?