If you live in NYC, my trick is to go to a gym that in on the way home from work, but not the same stop as my apartment. Then I just force myself to always exit the train at that stop even if I really don't want to go to the gym. Once I've stepped off and the train leaves it's either go to the gym or wait 15 minutes for a new train.
YMMV but I've never not gone through with a workout once I've stepped off that train.
YMMV but I've never not gone through with a workout once I've stepped off that train.