Last post I wrote that I felt any advice I'd have to pass on wouldn't work well, because of how long it took to really adjust. But in retrospect that sounded defeatist; I might as well pass on what I do know. Here's a few things.
Get away from the keyboard at least once an hour. Get up, walk around, go outside if possible. Don't just get up to answer nature calls or what have you. This is important. Your legs are you body's second heart, and when you move them you're promoting blood flow through the rest of your body. I also find that if I'm stuck on a sentence, getting up and pacing over to the other side of the house and back again stimulates me to come up with something.
Whenever possible, have an office with a door you can close, and at least one window in that office. If you don't, set up in a corner somewhere near a window. Being able to look out a window — not a screen, a window — from time to time is a boost to one's mental health. I remember working in an entirely windowless office early in my career, and when I was later moved into an office on the periphery of the building I was a good deal less depressed during the day.
Keep the surfaces of your workspace clean. This means keeping the number of things on your workspace low so you can wipe things down easily. Get under-desk organizers or shelving to help with this. Vacuum the workspace regularly, too, including the keyboard. If you keep snacks in the office, they should be the kind that don't create crumbs (jelly beans, hard candies, etc.) Also note that your monitor can become filthy, especially if you do any eating at your computer, even things that don't generate crumbs. (Saliva flecks stain screens.) For cleaning, I have a few packs of bar mops, which are more economical than paper towels. A pack of 6 costs about $15 but they last a long time, even with frequent washing.
Use a chair that promotes good posture. I spent money on a drafting stool with a footrest, which forces you to sit up straight. Likewise, spend good money on a monitor, keyboard, monitor stand, desk, etc. If you're going to spend eight hours a day typing and looking at a screen, make it as pleasant as possible. (This is why they nag you to spend good money on a mattress: you spend a third of your life asleep, and you want that to be good, too.) I have never used an under-the-desk balance ball or cycle, but some people swear by them.
Unless you take work calls on your phone, mute your phone when you're not actually using it. I have an app named Hiya that blocks junk callers and warns you if incoming calls are believed to be junk. It's done wonders for my sanity.
Not as relevant now for obvious reasons but worth mentioning for later: Some people thrive on working in a coffee shop or other such place; some abhor it. I abhor it, but experiment with such things if you do.
Whether you work at home or in a coffee shop, noise-cancelling headphones can be a great boon. My friend Eric reviewed a bunch of them, and the Sony models (earbuds, full-head model) seem like the absolute tops, but be warned they are not cheap. (Having a door you can close is a primitive form of noise cancelling.)
Eat actual meals. Meal-delivery kit services like Hello Fresh or HomeChef are biased towards two to four people, but you can cook for two one night and eat lunch the next day. Snacks aren't meals.
Since you're going to be using your bathroom as opposed to some common office one, buy a bidet. They take some getting used to, but even skeptics get convinced real quick after the first week.
Distractions and attention span issues are the biggest problems people face working from home. I have no panacea, just combinations of the previous suggestions, and a few other things:
Windows 10 lets you create virtual desktops, so you can isolate your work and non-work tasks to some degree. Mac OS has something similar, too.
Meditation (zazen) helps, but not immediately. It took years before it really started to have secondary effects for me, like reducing my distraction threshold. But it's never a bad time to start.
Two final notes. One, all of these things only work when done habitually. That's the hard part. Buying the right kind of chair or keyboard is easy. Getting up from it once an hour isn't. Get into the habit of making habits.
Two, none of this is dogma, just good ideas. I can guarantee you I have ignored all of these suggestions at some point. But I now follow them more often than I ignore them.