This is nothing revolutionary, but the number 20 has held special significance for me since I was a young teenager. I learned that if I started something and was still doing it 20 days later, it _stuck_.
Since good habits seemed to stick after 20 days, I started to set 20 days as a goal whenever I wanted to get into the habit of doing a good, responsible thing. Turns out, it works.
I think there are two reasons it works. The obvious is that, for me, 20 days is the right amount of time to internalize behavior or get used to a new feeling, thing, taste, activity or what-not. The less obvious but more important reason is that 20 days is short. It's easy to commit to doing something for 20 days. It's hard to commit to doing something forever. So, because I have this short duration which I believe to be like a magic spell, I'm able to convince myself to actually _try_ to do something hard. Even if something sucks, I can do it for 20 days.
The number 20 probably won't work for everyone, but I highly recommend trying it out and fine tuning the duration. Want to eat better? Exercise? Learn Japanese? Practice a musical instrument more responsibly? Try to do it _well_ for 20 days. That's it. Don't worry about whether it's going to stick. Just promise yourself the 20 days.
Even if the magic spell doesn't work for you, the worst case is that you do something good for you for 20 days.