I don't know what aircraft you're flying but if you can bring the pedals in a bit (or you closer to the pedals) this may help. I concur with the advice about the oregon aero products, never fly a 500 without it.
I used to get crippling sciatica flying Astars and a definite contribution was flying with my legs extended too much. It was almost an overnight cure when I went to a 500, even though I was flying up to eight hours a day with lots of leaning out. I reckon the much closer pedal position, even fully extended on the 500, played a huge part. The downside, however, is more stress on the knees. Especially if you're working the pedals a lot.
Depending on what flying you are doing, it's not a bad idea to, while flying, make a real effort to sit up really straight with your arse sticking out. Yes, it sounds pretty dumb and might look very strange but even just a few minutes of this every hour or so may help.
I also used a back brace for a while whilst flying the Astars and it seemed to help. Nothing too elaborate just fairly wide, 6" to 8" or so with a big velcro fastener. It was actually thin enough to wear under a shirt and you could certainly wear it under a flight suit. Keeps the gut in nicely, too!
Sorry, all over the place with that reply. The main point being I began to have horrendous pain and numbness, but have literally not had a problem in the last 5 years, (approx 3000 hrs). Good luck.