Hi Meryl. I agree with Jonathan and Harris; the best way to handle the situation that you describe is to have a ‘expiration date’ on purchased sessions. Of course, one of the drawbacks to having that policy is that you may find that your clients no longer purchase any of your longer packages for fear of losing money. So if selling those types of longer packages is part of your marketing plan that may be something to think about 🙂
I hope that this helps.