Take a suitable cushion to work, and take any opportunity to walk or stand.
Several of the chairs on the Comfortable chairs page are suitable for working at a desk.
If you work in the UK, your employers could get funding towards furniture to help you work.
If your work is at a desk, the best solution is a sit/stand desk or workstation - one where the surface can move up and down, carrying a computer. You can spend periods sitting, then move the desk up to stand while you work. Various kinds are available on the internet:
Sit/stand keyboard tray and computer shelf
I (Jon) use the sit/stand workstation below. It is made by European Furniture Group. It works very well. There is a lever underneath that locks and unlocks it, and you just pull it up or down to the right position. Weights inside compensate for the weight of the desk, computer, etc. In the second picture I am kneeling on an office chair which has had its back removed.
Rebecca Scudder writes: My work has been somewhat reasonable about accommodating for me. We talked about getting a sitting standing work station, and since they were not convinced it was the answer, we thought of a temporary solution to try the system. If your work has not yet gotten you the correct system, you may want to try this temporarily.
I sit or stand every few minutes for most of the day- my boss thinks this works well enough they don't need to go any further- but I do get a lot of tension in my neck & shoulder muscles- a distraction from my tailbone!
Llcasey writes: I have a "keyboard riser" which allows me to stand and work. Its made of styrofoam so it's very light and fits on the regular keyboard/mouse tray. Very cheap!! It's made out of some packing stuff a friend of mine had. It's four 2" thick pieces glued together that are the width and length of the keyboard/mouse tray bringing everything up 8".
I'm lucky enough to have a 21" monitor, so it's no strain to look at it standing or sitting, but most monitors do tip up and down. I also use it as a foot rest when I am sitting, and I use a couch cushion to kneel on sometimes, too. A kneeling chair is good as a change (see the second picture).
If a lot of your work is typing at a computer, then you should consider using one of the voice input systems. According to the reviews, these now work well enough to be a feasible option, provided you have a powerful enough computer, plenty of memory and a good (preferably noise cancelling) microphone. If you can arrange matters so that you can see the screen lying down, you can both input text and manage the normal controls of the computer by speaking.
I took a camp bed into the office before my operation, and I still keep it there. I alternate between standing or kneeling to write or work at the computer and lying down to read. To stop the stream of enquiries when I first got the camp bed, I put a notice up saying: 'Don't ask. I've got a back problem, but I'm managing fine, thanks.'
You can even lie face down to work, using a massage table. See Fritz's story.