Switched to paper notebooks after Excel crashed and wiped out weeks of tracking data.
I use one page per month in a dedicated notebook. Each invoice gets: client name, amount, date sent, due date. When they pay, I cross it out with a thick pen.
Every week I flip through and circle overdue stuff. Makes it dead simple to see who needs a follow-up call.
Writing by hand helps me remember way better than clicking through software. And it never crashes or needs updates.
Still send digital invoices but the notebook’s my backup tracker. Three years running now.