I've never found a way to make sure every day is guaranteed to be productive, I have stretches of being non-productive. It's ok, within reason. The most important thing, to me, is to reboot sooner rather than later. If you let it become later, you can lose a lot of time and not notice initially (the days will seem to slide by rapidly in hindsight). I treat it mentally like a small vacation, then tell myself to get back to proper work mode.

To do that, I pick small, easy wins, then roll from there until I'm back into a good day to day work routine of productivity. I've found making sure I do something early in the morning as part of being productive, makes a big difference in breaking slack patterns. Not letting the morning get away without doing something meaningful toward whatever I'm working on. I'm strongly a night person and I still find that to be critical. In my observation of low-productivity periods of time throughout my life, my mornings were always mediocre for productivity. As I've gotten older (closing in on 40 now), the value of being productive early in the day has gone up considerably (it's an energy conservation and utilization thing).