I used to be an after-work exerciser, but DD came along and that stopped working.
So now I wake up at 4AM! to get in a workout. At this point it's automatic... i think that it works for me b/c I make sure everything is out and ready to go, work clothes and lunch packed, etc - so it would take MORE mental effort to avoid it than to just get my butt out of the house.
Also, I keep my shower stuff for the gym IN MY CAR - which includes the hair dryer. So if I don't work out, then I need a shower, thus requiring me to go out to the car first. At which point I may as well keep going.
DD is so accustomed to having Daddy only in the AM, that it actually throws off their routine entirely if I am home. So avoiding a screaming, crying preschooler is a good motivator