It doesn't matter what area you decided to work on first, you need to be able to measure it. It sounds simple and hopefully obvious, but it's amazing how seldom it is done.
You might offer a discount to customers who bought product X if they returned within Y time and bought product Z and quote the ad or letter. You would, therefore, need to know who bought X within the time, and whether or not they bought Y AND qualified for the offer by quoting the offer.
That's a simple example - but applies to all areas - not just sales. For example, reducing overdue debtors, changing a product to reduce failure, and so on all require measurement. No matter what area you work on, you need to be able to measure the effect of the changes you make. Otherwise, you will never really know whether or not you've succeeded, and by how much.
These are often referred to as KPIs (key performance indicators). Different businesses have different KPIs - as do different parts of the same business. it is important not to use KPIs for the sake of it - and to end up with so many you lose sight of the goal. KPIs are not magic - simply a measure of something you decide is important to monitor.
Of course, it's not that simple. What if it doesn't work? Here's the second lesson in management: don't give up - at least not immediately. If you've got a problem with late debtors, there are a range of approaches you can use - and the chances of one of these eliminating all late payers are fairly slim - unless you choose to get rid of all customers who don't pay on time. That can also be expected to reduce your sales and profit - so it is important to keep an eye on the bigger picture.