I had an operation on my back, which got rid of most of my sciatica, but I'm guessing things are not that desperate for you, at least not yet.
I change posture every half hour, mixing standing, sitting, and lying down — and I have two different postures to lie down, a zero gravity recliner, and a bad. Also, I use two different chairs to sit, so I'm only using the same posture for one hour out of every five or six.
A chair that might be worth trying for you is one made by Carmichael. It's an office chair based on their drum thrones — I think they use the term "floating spine" to promote them. There is a gap through the centre of the chair, front to back, so the cheeks of your behind sit normally on the chair, but there's no pressure under your spine. For me, it's very helpful, as there's no pressure on the tailbone. Whether it will help with YOUR sciatica or not is anyone's guess — every back problem is different. But definitely worth a try, if you haven't tried one already.
Just don't do what I did, and go, "Wow, this is GREAT," and sit in the thing for four hours the first day you try it. The combination of muscles you use while sitting on this chair are quite different than any other chair, and later that night my poor back was cramping all over the place. Just take the time to get used to it, alternating with a different chair for awhile. Once you get used to it, it's a great chair.