Ah, the hardest question out there! There are two parts to the question
1) Finding your balance
2) Keeping it
You're asking about the second, but most people haven't figured out the first, so I'll start there with some thoughts.
In terms of finding your balance, you have to first know what does balance mean to you?
+ Hours: Does it mean X amount of hours per day? Does it mean X hours per weekday and Y hours on weekends? Does it mean reserving time for special events?
+ Special Activities: At McKinsey, an expression people used all the time, which resonated with me was "what brings you energy?" In other words, regardless of how many hours you work, as long as you have time for X, you'll feel great
+ Control over your time: Perhaps, regardless of hours, as long as they can be predictable, you'll feel balanced
+ Amount of travel vs. home.
In all of these, everybody is different and this can change over time, too.
Then there's keeping the balance:
+ Communicate it to your supervisors, managers, staffers, etc. and repeat it right at the start of a project
+ Work with people who respect your limits (this may depend on the firm you work for, so know what you're getting into)
+ Don't take crap from anyone, but be reasonable about rare exceptions
+ Overdeliver on your project work, so you never feel self-concious
I can tell you that, personally, at McKinsey, I was always scared to communicate my limits, but whenever I did, I was very impressed by the respect I received from my colleagues.
Happy to discuss further if you reply her or DM.