It’s all down to administering a good system.
What does good mean:
1. Easy to implement
2. Easy and convenient to follow- no system is good if it becomes to difficult to stick to.
3. Takes the least effort and time. If you have a system that takes too much effort, you will naturally not stick to it.  We’re all busy in some way and all naturally adverse to cumbersome admin (who likes admin?).
Excel is a great tool- whether you print a spreadsheet (suggestion is to keep it to 3 columns maximum: choose 3 of these:  date, description/client, amount or reference). Update either by hand (very easy) or on your computer. I find having a physical sheet on the side of my desk the simplest. It takes 3 seconds to update- no excuse. Nothing fancy, it just works.
Clipboard (or paper spike) method- 2 clipboards: 1 for payments and 1 for invoices. In the last week of the month file those which have been paid/received and keep those which are outstanding to take action on. Mark on the document also when paid even by writing just the date (helps when you need for records).
Accounting/billing software (or app) can also do the trick. Its all personal preference.
Diary! Once I put a file away I add a reminder (Google/Outlook or whatever) to pop up when I want to follow up on it next. This takes 4 seconds and it then takes the file off your mind until you need it.
Make sure all relevant information is at hand, so if you take down debtor information be sure to include file notes where necessary and contact details (writing it once will save you from having to search for it later!).
Always act fast , nothing weighs you down more than tasks which drag on. And most the time they can be sorted out relatively quickly.
Don’t let invoices stay outstanding long Send letters to customers who don’t pay
As hard as it is in our busy lives- a drop of discipline and order will set you miles ahead.