In addition to a diagnosis and getting the right meds, and all the work / task management habits, I’ve found that meditation and mindfulness has been really helpful for calming me down, and letting me focus on what I need to focus. When I meditate there is usually an initial startup phase where my mind is racing, from one worry or task to another. I consciously try to acknowledge the presence of the task and then mentally tuck it away and try to regain focus on my meditation object (the breath leaving my nose or the breath in my belly for example).
This ability to step away and consciously not engage with a worry or idea is kind of the crux of the ADHD problem for me - so meditation helps. It helps me remember to “let go” and be okay with not focusing on that worry, so I can get back to the central goal sooner.
Many people have mentioned organizational tips like lists and task management. I’d expand to offer systematic organization of physical things in my home to be similarly helpful. The idea follows that when everything has a place (this book goes here, that cable goes there, etc), and you know the place or system rules, then it’s easier to recognize when things are messy and fix the situation. Ive still got a lot of work todo here but that Marie Kondo Art of Tidying Up show helped me for sure.