We have all had those appointments that if you don't show up, you will be charged a fee. While it can be annoying, I never miss an appointment with a cancellation fee. No matter what.
Why not think about treating your scheduled workout as an appointment with cancellation fees. You wouldn't miss if a personal trainer were there, so respect yourself as much as you would a trainer.
If you cancel a scheduled workout (in your mind or on paper), then penalize yourself with something. Nothing too big, because you don't want to be too hard on yourself (things happen), but perhaps something small such as no dessert after dinner or no TV.
My friend and her husband penalize each other with a 'cancel jar' and every time one of them doesn't work out when they said they would, the one who cancels has to put $5 in a jar. At the end of 6 months, the person who has canceled the fewest amount of times gets to spend the money however they wish.