Promote your business! Create a series at a special reduced price to bring in and keep clients. But make it available to only clients who have purchased a different initial series. You can do this by:
1) Setting up a membership that has a discount on services built into the options
2) Making the initial series the client needs to buy in order to be eligible for the special a membership type of item
3) Marking the special series as available to members only. That way, only clients who have purchased the qualifying initial series will be able to buy the special. Also, put a note into the Receipt Notes of the qualifying item so your clients are aware of the deal! And however long the duration is on the qualifying item, that's how long they'll have the opportunity to purchase the special (i.e. until their initial series expires).