Big changes or small changes?
There are two ways to make a major change in your life. One is to make a series of small changes--this month you might brown-bag your lunch one day a week; next month you might go for a short walk every day. The other is to make all your changes at once--cook all your own meals, exercise daily, and turn your hobby into a business. Either way can produce permanent changes for the better.
There are a lot of advocates for the strategy of making small changes one at a time. Practice one good habit regularly, and soon enough it will be a comfortable part of your life. A year's worth of such improvements can bring you to a very different place than where you started. All it takes is a willingness to try, persistence, and a little patience.
I think, though, that the strategy of making big changes has its place. I see it as being appropriate when big changes are urgently needed, and especially when big changes are inevitable.
Piggyback on inevitable change
Sometimes your life is simply going to change--you're going away to school, you're changing jobs, you're moving, you're getting married or divorced, your child is being born or going away to college. Times like these, many things about your life are going to change anyway--why not be strategic about it?
When you have to make a whole new schedule anyway, you have a natural opportunity to make big changes. Such a thing is always stressful, and the natural response to stress is to seek comfort and familiarity. Use that to your benefit.
Take a few minutes to think about the things that give you comfort and pleasure--and then think of which of those things are good for you. Make a list of comforts that are healthy and cheap--phone calls with old friends, long walks in the evening, visiting the library, reading the Sunday paper.
Then, when you make a new schedule to suit your new circumstances, go ahead and make an ideal schedule--include all the stuff you know you should do and leave out all the stuff that you know you shouldn't do--but be sure to include an abundance of things from your list of comforts. (The birth of a new baby or starting a new job might seem to leave little or no time for simple comforts, but you can almost surely make time for some by leaving out the things you want to quit doing.)
Will it work? Maybe not. Maybe it won't work at all, and you'll have to go the slow route, taking small steps. But if you try the experiment when your schedule is all up in the air anyway, I think it's at least safe--you're not likely to end up further from where you want to be than you started. You're not even likely to lose any time that might have been spent adding one small habit--it's probably silly to try to make one more small change when you're already in the midst of major changes.
If you're not in the midst of major changes, it's more of an open question--because then you are losing time that might be spent making small changes. It could still work--people make big changes in their life all the time, even if many try and fail. If you want to make big changes, and if you've never tried--or haven't tried in a long time--it may be an experiment worth trying.
If not, small changes are another way to get where you're going. Not even a slow way, necessarily. Small changes can add up fast.