i have been working at mineral water company for about 6months and it hardly for me to get a nice strategy how to get back our gallons from customers as 1 customer mite have 2 till 5 gallons at their home n they will order our gallon again after they have finished all of them.

My problems here is how to make them will call us back before they have finished all of their gallons so we can use our gallon for another customers again.as we had more than 6000 gallons already but still not enough.

is there any strategy for me to do a great return of our gallons from customers before they have finished all of them?. thank you in advance.