Frequent Buyer program?

For our PlayOn program (single ticket buyers between 16 & 29 years old) we'd like to encourage them to buy more tickets and are interested in starting a frequent buyer program that would tally their ticket buying within a season and give them the 6th ticket free.  Has anyone developed something like this that tracks buying across the season (not just within an order) and then applies an offer based on these purchases?

Any guidance would be appreciated!