yup, calling every day was the only way I managed to get past due payment I was owed in the past. If they hedge (oh it's in our internal systems) ask for a day to call them back and call back the day before. For my cases it's usually been someone different handling the finances side of things, and the customer who I dealt with is actually embarrassed payment hasn't been made.

You could also start charging interest on the overdue bill, but I believe you have to state this in your invoice. then you have an excuse to call - to update them on the amount they owe.

I've not heard of anyone at startup level resorting to a collections agency but I have heard of the small claims court being involved sometimes. Never had to go that far myself, so I can't really comment.