I've been using the Chrome extension I built [1] to set a task and block distractions that aren't relevant. This turns my browser into a less distracting environment which is a big help.
Even more important is setting the right physical environment - I'll try to find a relatively quiet coffee shop that doesn't play music (or at least not pop music). This is still fairly suboptimal though, especially because I am addicted to people-watching. After a couple of hours I usually find myself observing the facial expressions of baristas as they go about their job. The best working environment I've ever found was when I used to sneak into the psychology experiment cubicles at my university - small, isolated white rooms with no visual distractions.
Taking a small dose of dried psilocybin truffles (about 0.25g) can be quite helpful for focus. It puts me in more of a third-person view where I'm less likely to get distracted by external things although it also makes me more emotional which can sometimes leads me to get distracted by internal things.
I've used modafinil in the past which definitely aids focus (I think partly because it alleviates depressive feelings) but the long half-life ends up affecting my sleep and it becomes a negative trade-off in the end.
Morning is definitely the best time for getting work done. I find it very difficult to reach the same level of focus after lunch so I try to squeeze as much as I can into the first four hours of the day. This might be related to nutrition so I could do with experimenting with my lunch diet to improve this.
To-do lists and calendars are helpful to a point but I'm so bad at sticking to them that I often end up feeling deflated.
[1] - https://deepmode-website.herokuapp.com/