The most important thing to do to stick to an exercise routine is to schedule that time into your diary just as you would any other meeting or commitment. Cancelling on that time will then seem just as rude as cancelling an appointment or meeting at the last minute with someone else.
Write it in your diary, on the weekly planner on your fridge or desk, insert it into the calendar on your phone or if you use an app to plan your time – include it in there. Treat this time that you will be improving your health, fitness and strength just as would any other scheduled time. Set yourself reminders, use electronic notes, stick post-its on your screen, ask your colleagues to remind you before you leave work to stop at the gym on your way home. Do whatever you need to make sure this allocated time is important and productive instead of incidental and easy to cancel. 🙂