If you have sciatica as a result of Piriformis syndrome, this is one of the better sciatica stretches to do.  Piriformis syndrome is the condition where the piriformis muscle, located deep in the buttocks region, will have spasms, become tight and possibly apply pressure to the sciatic nerve.  This muscle is responsible for rotation of the hips and legs.

Start off by lying down on your back with your legs slightly bent.  Cross one of your legs over the other.  If you want to stretch the right side, put the right leg over the left, and vice versa.  Now, lift the bottom leg up as far as you can, using your hands to help lift the legs and hold them in place.  If done right, you should feel a nice stretch in the buttocks.  Hold this stretch for 15-20 seconds, doing three different sets.

You can also do this stretch without having to cross the legs fully, which may be uncomfortable for some.  You can lift one knee into your chest and stretch it across your body, leaving the other leg on the floor.  This method does not give as good leverage as the first method detailed and you may not feel as good of a stretch.

Alternatively, you can do this with you legs on the ground, with your body upright.  Watch the video below to see what this Piriformis stretch looks like: