Many people succeeded at this by going to bed early, waking up early and doing their things (developing, learning, whatever) in the early morning. This way you do so when you're still fresh!
I have to second this. I used to feel guilty about relaxing in the evening after work. Now, I make a conscious decision to relax in the evening, go to bed slightly earlier, and wake up in the morning to work on my side projects.
Personally, I get up around 6:00 every morning and get a few hours of personal work done before I leave for the day job. I find it so much more productive to do creative work in the morning when I am fresh and alert than in the evening after a full work day.
Also, the added benefit of avoiding social distractions helps a great deal. I know I'll be able to sit down by myself, with my morning coffee, and have several hours of highly focused hacking.
This doesn't really work when a longer commute is involved. The morning (without waking up early) starts at 6am already for me. I suppose it works differently for everyone, but I have way more energy/focus immediately after work and late at night. I guess it just depends on if you are a morning bird or night owl.
