I agree with both of the above comments. I believe that you need to know what will work best for you personally (acknowledging that this will also change, and that is ok). It is then feeling confident to implement these when looking for, interviewing and accepting a role. To have these boundaries in place will mean you can ensure, to the best of your ability, you are working somewhere than can enable you to achieve the "work/life balance" you are hoping for.
Understanding what an employers key hours are, combined with what hours of the day you work best within. For example, if you were to work 10-4, but then recognise you'd rather complete tasks again in the evening, from 7-9, or alternatively, you are a morning worker and would like to work earlier.
Building a strong relationship with your line manager, which can take time. Often once somebody has past their probation more flexible working opportunities are available and can be utilised.