I can tell you how I do it, but I wouldn't necessarily recommend my path to anyone else. I'm not even 100% sure I'd recommend it to a younger version of myself, although I'm also not sure I'd recommend
against it either.
Basically, I'm single and childless, and darn close to my every free minute goes into the startup project. When I was working full-time, that meant "after work" M-F and the majority of my Sat and Sun. hours. I make an exception during football season to watch the Dolphins game on Sunday (yeah, yeah, go ahead, say it... I'm a glutton for punishment).
I force myself to take a break every once in a while and read a novel or watch a movie or something... but truth be told, lately, even when I watch movies or something, there's an angle related to the startup. For example, I've been reading and studying a lot about sales lately, and I've seen more than one sales book mention "be like Columbo".
I had kinda wanted to rewatch some old Columbo episodes anyway (yeah, I'm old, deal with it) so I've been watching Columbo and thinking about "how do you apply his attitude / approach to this 'diagnostic business development' thing"? And the movies I make time for tend to be ones that I find inspirational (Rocky, The Karate Kid, Rad, etc.) that help keep me motivated.
About 6 months ago, I dropped down from full-time status to part-time status at my regular job to free up more time for the startup. I had a little bit of money saved, but not enough to quit outright... luckily the owner of the company was willing to be flexible and let me do that. I've pretty much burned through my savings though, and we aren't profitable yet, so I'm going back to working full-time next month.
These days I forgo most social activities like dating, parties, and most non-technical social events. I rarely mountain bike anymore, and almost never set foot in the gym. I bailed on going home to see my parents for Christmas so I could use those few days of "down time" as recovery time. I stay amped up on more caffeine than could possibly be healthy, and do everything I can to the tune of heavy-metal[1] or hard-rock of some sort. My mantra right now is "Caffeine, heavy-metal and adrenaline". \m/
I know, I know... you're thinking "dude, you must be miserable". Nah... I enjoy this in a certain sense, and since I'm 40 now, and haven't achieved anything yet, my mindset now is to say "fuck it" and go for broke. It's now or never, ya know? My mindset is to achieve the biggest and most ambitious of my dreams, or - when life finally breaks me - pull a Leaving Las Vegas deal and go to Vegas and try to drink myself to death.
Do not try this at home...
[1]: https://www.youtube.com/watch?v=KQQcOQsCFnw