Make an appointment to exercise
Here's a trick to try:
Even if you don't have a personal trainer, you should make an appointment in your schedule for your workouts.
Think about your workout appointments like making an appointment with your doctor.
You wouldn't just skip an appointment with your doctor - so don't skip an appointment for a workout (and in the process avoid having to make a future appointment with the doctor by staying fit and healthy.)
Take it a step farther:
Try having a penalty if you skip the workout. For example, if you miss a doctor's appointment you usually get charged a fee. Pick an amount and if you want to skip an appointment you have to send the money immediately (either online or via cheque) to an organization you do not like and do not want to support.