One habit a month...
A few months ago I stumbled upon a Hacker News comment that was, well, not life-changing, but it could be on its way to become so.
Here's the meat of the comment:
Design an ideal day. Better yet, design the day you want your 27 year old self to have. Now, you know what your 27 year old self does with his time. You've got 2 years to become that guy.
This means you make gradual changes to your habits. The 30 day method works well for people. Pick one of these new habits that you're aiming to have - only one, seriously - and stick with it for 30 days. Then keep that one going, and add a new one.
If you can do that, you can introduce 12 new habits in one year, and 24 in two years. That's an enormous difference, and it's entirely achievable.
I started out with a few silly habits, mostly things to do around the house, nothing spectacular. But after 3 months, the effects are already noticeable.
There are a lot of things I want to do and learn, and I realized that using this method I can do them and learn them. For example, for November I am starting a simple study plan; the idea is to spend 1 hour every evening. If this works, and I can keep it up, I can gradually add more time for studying, and other things as well.
(I am deliberately not setting a strict schedule, because that might not work in the chaotic environment that I live in. If a day is shot to hell for whatever reason, and I don't get to study, no sweat; I'll just do it the next day. This isn't homework. :-)
Also, it's nice that there is little overhead. You just keep a list of things you do every day, and add one each month. If you keep these things small, it will be easy to do them every day, they become a habit (as intended), and then you just let the results accumulate. (For example, if your place is constantly messy, you can add a habit of cleaning up just a little bit every day. After a few months (or maybe even weeks), everything will look a lot better; and this way you won't get overwhelmed trying to do it all at once.)
I do have a bit of trouble sticking to one new habit a month... I sometimes want to add too much at a time, which is going to backfire if I let that happen. It would be like going to the gym when you're completely out of shape, and immediately trying to lift the heaviest weight, or trying to run ten miles. Not only is it not going to work, but you're going to hurt yourself in the process.
Even if the habits themselves aren't that impressive at first, they do give your day some sort of structure. (This is especially useful for me, since I work from home, and my hours are fairly flexible, so there's not as much of a daily routine as with an office job.)