If it had just one cause it would be easy to fix.
Unfortunately, it has many causes. The Wiki article on sciatica is a pretty good start.
Mine was a combination of them - spinal injury and resultant muscle spasms putting pressure on the nerve.
Given that yours starts in the buttocks, I would suspect that it is either the Piriformis muscle in your lower back and/or the Gluteous Maximus muscles fighting it out. If so, check into some exercises to stretch and relax these muscles, relief should bring almost instant relief.
A good Physical Therapist can help as well.
(Ping to neverdem for any additional help/comments.)
I still get muscle spasms from time to time. Just had a bout of them this past week. Regular stretches designed for back support are good. Stretch the butt, the hammies, the lower back. Nothing too strenuous. Take Ibuprofen as needed. Heat is good. And a good massage from a licensed therapist helps a lot too. I stay away from chiropractors. I find them a little rough.