The following are a couple of tricks I've learned to ensure that I get a good workout whenever possible:
1) Always have an extra set of workout clothes (top, shorts, socks and sneakers) in a spare gym bag in your car. That way, if you wind up having time to go to the gym, you don't have to worry about forgetting your gear.
2) Have a spare pair of running gloves, shades and ponytail holder in your glove compartment.
3) Tell your friends, colleagues and family that you are going to workout. By being open about your plans, you are committing yourself to working out.
4) Always have a water bottle with you, whether you are at work or en route somewhere. Staying hydrated will keep you alert during the day and you will remember to workout as well.