Other than yourself of course.
For me, my biggest saboteur is my husband. The funny thing is though, he doesn't mean to be. I don't think he even realizes it. He's been better this time, but in the past he would bring me "treats" to reward me for doing a good job. For example he would bring home pizza's for dinner, and a cheesecake for me, since I've been "so good". I've had trouble in the past giving up on the soda, so he would help me by bringing me one home only a few days a week. Then a few would turn into one every morning, which would turn into me just drinking soda all day long. It wasn't his fault, I chose to drink them, but the temptation was there.
This time it's the exercise. I was mentioning that it was a little harder working out on Saturday, since the oldest two kids are home from school all day. They normally play outside just fine, but it's just two more to listen for. He's answer was "well, it's ok if you only work out 2 or 3 days a week, it's something". He has been much better this time around though, I have to give him props for that. So far there have been no treats, and he asks before he gets me anything to eat.
My biggest motivator oddly enough is one of my 4 year old twins. I find that the easiest time to workout is during naptime for my 2 year old (and if I am really lucky, my 7 month old ) while my older two are in school. That leaves me with just the twins. As soon as my two year old goes down, Joshua starts with "mama, it's time to go exercise. Come on mama, let's go get strong." He's very insistent until we go downstairs, where he'll either play with the baby or run around while I work out. All my kids are into hiking as well, so it's really easy to get them out and about when the weather co-operates.
So how about you?