Don't let either your personal life or your work life go neglected.
There. That's it. If you are short on time, you can stop reading now, since that's the one most important thing about work / life balance.
Oh, you're still here? I might as well talk about the actual balance part, then. The part before was just about boundaries, but within those boundaries are many possibile ways to allocate your time. I'll try to keep this advice short too: Do you what love, be flexible, and keep learning.
"Do what you love" is pretty standard advice, but most people use it to talk about your career. It applies to your extracurriculur activies too. It means that your time on nights and weekends is best spent pursuing the priorities that are important to you at the moment. For an engineer right out of college, that probably is some combination of working and hobbies. For an engineer with a family, that is mostly going to be family activities and hobbies. That might be family activities and work, but family activities, at least with young children, are a fair bit of effort. I think hobbies balance out better with family life.
Lets assume that you have a family, since that is when people start thinking about the work / life dilemma. I think it is completely reasonable to work just 40 hours a week. However, you have to make sure that those 40 hours are spent wisely. No wasting time at work. In fact, you have to make you are always working on the highest priority task, keeping your focus, and not going to unimportant meetings. If you do those things, you should do well even if others are working 12 hour days.
"Be flexible" is just saying that you shouldn't have any hard rules about work / life balance. Sometimes you do need to take extra time for work, to fight some emergency or to finish off a release. Or sometimes you need to take less time than normal for work, so you can tend to issues that come up at home. If you have an urgent family situation, your work should be flexible and let you take time off. And if you have an urgent work situation, you family or other non-work life should be flexible and allow you to work that extra time. So what I said before about not letting work or personal life go neglected is not necessarily true for short periods of time. It's only true for the long run.
"Keep learning" is about the reality that there is a bit more you need to do away from the office. You need to work to keep your skills fresh. That means not only devoting time to learn new technologies, but also reviewing the fundamentals. The good news is that this doesn't take a lot of effort. I'd guess an hour a week of study or experimentation is sufficient. More is better, of course.
I've seen too many people, both old and young, stagnate because they only did their work. It isn't enough, because change is constant in this industry, and fundamentals are easy to forget. It is good if you can respond to changing technologies, but even better if you can lead some of that change. Keeping your fundaments sharp and keeping up with new technologies will put you in a good position to do either.
Finally, I'll leave you with a piece of concrete advice: Don't check email within three hours of your bedtime. It raises stress levels, and on receiving mail, you are often compelled to act on them or reply to them. Even if you choose to be working in the evening, just work, and skip the email.