1) Taking Modafinil[1][2] semi-regularly. I've always had problems focusing and being productive. Honestly I've always wondered if I have some form of ADD or something. I used to drink large amounts of caffeine but the effects after long term use declines dramatically due to tolerance and I found out I was lactose intolerant. So I tried Modafinil after hearing a lot about it. I personally find it to be an extremely useful tool for my productivity and even my social life. It's also a much better drug in comparison to caffeine (disclaimer: individual results may vary, do your research, I'm not a doctor, etc).

2) Write things down. Make a list, write a journal, what ever feels right and works for you; just dump thoughts or ideas from your mind onto paper. Make it as structured or unstructured as you desire. It might be projects, things you want to watch or buy or upgrade around the house, etc. My 'notes' are literally completely random things I've thought or been told immediately and want to remember later. Most of them I'll never see again, but I know they are there. Eventually some of these might make it into the next step.

3) Get some productivity/task software or devise your own system for tasks and reminders and actually stick with it. I use OmniFocus2[3] but use what ever works for you. A simple calendar and notes system would work for most people. I work from home and found myself just reverting to reddit or hacker news when bored, where as now I see if there's any tasks assigned for today and what needs doing and do those before I allow myself to relax for the day. Having my day already planned makes me so much more relaxed when I wake up. The alarm can go off and I don't have to worry that I forgot to email my client, or the tax statements need to be mailed today. It's already sorted out for me to do and I don't have to think about it. It frees the mind to think about what you're focusing on, or a problem that requires devoted attention.

4) Every few weeks or so, spend some of your down time reflecting and thinking about what you can automate to save time, or what you can change so that you're more productive. Automate and iterate upon your productivity systems like you would as if practising agile. You wont get it perfect the first time around. I identified tracking the time spent on different clients was taking a lot of my time by hand, so I automated my timesheets into a CSV format which is imported into Google Sheets and used in my books for invoices. Later I automated the invoice email too. I used to do that by hand. I also created a filter that forwards my client's emails into my OmniFocus inbox and I can set the tasks for a date which appears in my calendar with alerts. You can do a lot, even with out the expensive productivity apps.

5) Don't work against your own unique circumstances or biology, work with them. I have a sleep disorder[4] (unrelated to the Modafinil, I assure you) and had a really bad time working full time normal office hours. I lasted two years full time waking up at 7am but I was getting 3-5 hours sleep and as a result I got burnt out, depressed and anxious. I quit and currently contract remotely earning a lot less money than I could otherwise, but now I get to work during my own sleeping hours and I feel a lot more happy. I tried changing my sleeping pattern and tried a bunch of band aid solutions but I never had any success, where as I've had great success adapting my life around my biological abnormality.

6) Reduce friction. It's really obvious when you think of it. I learnt this when I was playing guitar. If I have a guitar sitting next to my desk already tuned and ready to play, it results in me playing guitar more regularly. If it's untuned and in a case, I'd play it much less regularly. Reduce friction required for you to get started.

7) Don't believe anyone who tells you they have the solution™. You are a unique individual with unique problems which require unique solutions. This applies to self help too. No one has the 'real' answer. You need to try, experiment, think, and decide for yourself. Modafinil might kill you. Notes might waste your time. You might spend $80 on a productivity app and never use it after the first week. You might be suited to orderly enforced sleeping patterns.

EDIT: Formatting, added #6, and added some references

[1] Wikipedia Modafinil: https://en.wikipedia.org/wiki/Modafinil

[2] Very informative write up about Modafinil from the lovely Gwern: http://www.gwern.net/Modafinil

[3] OmniFocus2: https://www.omnigroup.com/omnifocus

[4] Delayed Sleep Phase Disorder https://en.wikipedia.org/wiki/Delayed_sleep_phase_disorder