Give your clients the possibility to subscribe to your products or services, with having to place a new order everytime, and you can also give them a discount so that they are more likely to do so. Doing this allows you get create recurring orders for your clients, and you make their lives easier if they need your product regularly.
The ReCharge application is recommended to launch this feature because it's the best integrated solution, and the most trusted. They do all the heavy work of charging customers again, and warns them when their products are shipped so you don't have to do anything on your side : their new orders show up as regular ones in Shopify.
If you want to take it further, a custom development can for example allow your clients to manage their subscription by SMS, pause, cancel or upgrade their subscription, without even having to log into their account, straight from their phone !