I also find that the biggest key (and hurdle) in keeping with a plan is scheduling.
Everyone has different work schedules, commitments to school, children, parents, sports and hobbies, etc. In addition to finding the right fitness plan(s) that works for you, it also needs to fit into your daily routine and lifestyle so it becomes part of that routine.
For example - I work full time, volunteer time in my town and I am a father of two kids under 8 who both have many extra curricular commitments of their own. We are an extremely busy family. We like to travel when we can and we like to eat out every Friday night. One would think that would make it difficult to find any time to build a fitness and diet plan, but it is actually the opposite.
Fortunately there is a gym in my office complex. I utilize that every day on my lunch break and either take runs or hit the weights for 45 min. If I'm busy at work and cant do that, I run in the morning before I go to work or on the weekends, or rarely after work. The key to it is keeping a plan that is flexible in weekly increments so you can roll with the punches and switch days if necessary.
I also play hockey on Tuesday and sometime Thursday nights. Those games are late enough to not disrupt dinner or any other things my wife and kids have going on. (God Forbid!) I build my diet around all of it and while that is mainly the most difficult challenge I try my best to keep a balance with it all.
I so easy to make excuses to not exercise and keep yourself in line. But where theres a will theres a way. Picture yourself in the body you want to be in and make it happen.