What should we do If we feel weak or sick? Should we traind and eat regularly?

I have several friends who have come to me for fitness advice, always prefacing it with "I want to get in shape...I just wish I had your discipline!" Then they tell me they'll workout more, they're committed this time....only to fall off the wagon after a few weeks. Do you feel some people confuse discipline with motivation or use them interchangeably? I am not always motivated to train, but it's a habit and a priority. How do you convey that and how do you encourage people to stick with it until that habit/discipline forms?