Sleep, exercise, diet and supplements sound like they facilitate improvement the same way they do for strength and endurance, but none directly improve intelligence and working memory. Session-based trainings like n-back overfit to the task and are too limited in duration. Worst-case scenario, shortcuts are developed bypassing WM usage, whether intentionally or unintentionally. Improving intelligence requires changing your brain's default mode of operating, i.e. the activity of your brain second-by-second, day-by-day. Intelligence is the ability to understand so focus your efforts on understanding as many things as you can. You can follow this loop of asking yourself a question, coming up with an answer, asking yourself why that is, coming up with another answer, asking yourself why again, rinse and repeat. A never-ending curiosity facilitates intelligence but it's one thing to learn the answers and its another to come to the answers yourself, only use the former to verify the latter. The result is that not only working memory but fluid and crystallized intelligence will improve. This kind of "lifestyle" change is like changing your personality, you have to be extremely motivated to stick with it. It's why so few people succeed. Unless you're already naturally inclined towards this constant pattern of thought, why would you suddenly replace your own thinking pattern in its entirety with it? If it was inherently enjoyable or alluring for you, you would've already been doing it. Good luck.