Well, I have the inattentive subtype, without hyperactivity or much of the associated impulsivity. So I guess I'm already in a starting point that is easier to compensate for.

I pay a lot of attention to my sleep - I'm a chronic insomniac. And keep in mind that it's not just about sleep time, it's also about sleep quality. So sleep hygiene is really crucial. No caffeine after lunch, and no alcohol in general (unless you know you can afford to be less well-rested the next day) because that might make you drowsy but it prevents entering the deep sleep cycle. Low-dosage melatonin seems to help me, not so much with falling asleep as it does with ensuring that once I do manage to sleep, I hit that "deep sleep" cycle that is absolutely necessary to recover.

I'm very sensitive to disturbances at night, noises can drive me nuts. So I spent a lot of time finding comfortable ear-plugs. They are essential for uninterrupted sleep for me, which again is crucial for hitting the deep sleep cycle.

Then there is the question of falling asleep. If I have trouble with that (which is almost every night), I get out of bed and go read in the living room, so that my brain keeps associating the bedroom with rest. Sometimes I start having ideas just before falling asleep, I then try to write those down on paper, and maybe work them out a bit - I realized that part of why late night thinking keeps me awake is that I'm afraid to forget all the ideas, so writing it down gives me peace, and doing so by hand also calms forces me to slow down. Meditation is also good for slowing down racing thoughts, but it really depends on what the thoughts in question are.

Then there's diet. I don't know if I can generalize what works for me specifically, but lots of leafy greens in general. There is evidence that berries and other sources of polyphenols help a lot with children with ADHD, and in general those are supposedly really healthy, so I figure they won't hurt me either. Avoid things that give sugar rushes/crashes. Finally I take supplements for common deficiencies that apparently disproportionately affect people with ADHD (B, zinc, magnesium).

Exercise helps because the cerebellum is involved with regulating attention. So I try to start the day with some mild exercise (like a short run) to really activate it. I've noticed that this helps with my attention regulation throughout the day. Sitting all day programming reduces activity again, so sometimes I take short "activation breaks" where I just do ten squats or burpees or whatever, not to exercise but to wake up the cerebellum.

Therapy is important. The overlap between ADHD symptoms and co-morbid/complex PTSD symptoms is large, leading me to believe that maybe people with ADHD are just more likely to end up in traumatizing situations (in my case: other kids loved to provoke temper tantrums in me) and part of their symptoms are really just a consequence of the resulting PTSD. As difficult as it was to process my past, therapy helped me a lot. It seems to have reduced some of my ADHD symptoms a bit, but even if it didn't: someone with ADHD is likely to have a lot of pain to process as a result of their condition. Doing so means having one burden less to carry, making it easier to cope with the remaining issues.

One very important thing I learned: ADHD means that inevitably I will slip up at some point and forget an important appointment or something. The screw-up itself is punishment enough, so if I hate myself for always screwing up too I'm just punishing myself twice. So learn to be kind to yourself and forgive yourself when that happens, that makes it easier to pick yourself up, fix the problem, and move on when this happens.

Therapy also helped me with being better at being brutally honest about my limitations. No, I cannot plan five things in a day when I know that I can't complete more than two. If I know I'll likely forget something at home when I go somewhere, even with checklists, leave early so I can go back to pick it up when I inevitably remember I forgot it five minutes after leaving the house. Try to develop good habits. More importantly, only try to develop one habit at a time, and give it enough time to really become a habit. And so on.

And finally: use that ADHD hyperfocus to your advantage, and do deep dives into learning as much as you can about how to ADHD works and what you can do about it.