Start new clients on a pay up front basis. You will need to perhaps sell a package of training sessions, with discounts for more sessions bought. Your new clients will have this new agreement, in this way you have the leverage to charge or not charge for late or missed sessions depending on your signed agreement. Clients tend to respect your time more when their money is involved.
You could also draw up a short new agreement for present clients, with a 24 hour cancellation clause and have them sign it. It is not that you will have the leverage to enforce it without the money, but at least you are creating more mindfulness of the issue.