I do it with accordance with a to-do list: setup realistic deadlines for all projects/side-projects and build a schedule based on them (The deadlines are the only "tight" stuff in my schedule, the rest is flexible). Your work/life balance has to be part of whatever you plan! (e.g. I usually try to have ~2 weekends in the month for other activities and ~2 weekends for projects).
Also it helps me during my free weekend/night to simply keep away from my laptop, this way I don't fall in the trap of wanting to work 1 hour and end up working the whole day.
Sadly your problem is quite common. I know too many people that can't handle their work/life balance...