I would maintain notes under each invoice as to where you are in the collections process. And then refer to the invoices report to see what you still have to collect on.
And to systemize things a step further, come up with a strategy to avoid collections as best as you can. I don’t know what industry you are in, but we take credit card numbers for all residential clients and it is required to get on our calendar. This way if they don’t pay us upon completion of the job we auto charge the credit card. Hope this helps.