If your goods (not services) remain easily identifiable as supplied by you, you should have a condition of sale which makes the goods your property until paid for. You may then recover them at any time, but especially when a customer is in difficulty.
You should notify customers of the new clause and make sure it is simple but clear. An 'All Sums Due' clause allows you to recover your goods against any monies owed to you, not just the debt for the goods recovered.
Retention of title clauses are not appropriate if your goods are to be processed or incorporated into something else. They also need very careful wording so they are appropriate to your business. It is advisable to seek legal help in drafting.