Attendance is just as important to track as sales. After you sell someone a subscription, you don't want to become complacent. So you have hit your subscription goal this season. Congrats! But now comes the hard part. You need to track which of your subscribers purchased subscriptions but are not attending. These are the folks who will not renew for the following season.
Many people have asked me how Virginia Stage Company maintains such a high renewal rate (last year was 89%). I am certain that one of the reasons we have a high renewal rate is the constant interaction we have with our customers. After each production, I run an attendance report showing which subscribers did not attend. If I see a continuing pattern starting to develop, I contact them either through an email, or telephone, to just talk about their experience with VSC. I don't come on too hard, and usually they tell me right off the bat why they haven't been attending. If it is something that I can correct, or address (and about 90% of the time it is), then I will go out of my way to get them to the theater. I am sure that once they start attending regularly, they will love the experience.
Maintain that personal contact with subscribers who for some reason are not attending. They will appreciate that you are reaching out to them, and addressing their "problems" in a timely and attentive fashion.