Pressure on the sciatic nerve can be caused by different things, such as compression around the spine (muscle, bone, slipped disc) or other muscle compression along the length of the nerve. Often the problem is caused by a tiny butt muscle called "piriformis". Piriformis is commonly strained (therefore tightened) by activities in which the body is rotated on it's leg... simple things such as repetative rotation while washing dishes (wash-turn- rinse-turn- stack-turn) or shoveling, or similar movements. The sciatic nerve is big, about as big around as your thumb, and it runs alongside or even through this little muscle.
It is fairly easy to discover if piriformis is the source of the pain. If you place the pad of your thumb on your hip bone (on the painful side of your body) and stretch the rest of your finger back towrd the central part of your buttock (the fleshiest part) your middle finger tip will be hovering over piriformis, aproximately! The muscle is deep, well concealed under the large muscles, but if you apply pressure in the general area you will likely find a VERY tender spot (IF that little muscle is , indeed, the culprit.)
If you have access to a massge therapist he/she should be well versed on this subject. If not, it is possible to self-massage pretty effectively by lying down on your back with a tennis ball pressed into that spot, or even carefully backing into the corner of a counter (might want to pad it with a towel) or other stationary object to apply steady pressure to that muscle.
Oh, it will hurt! That muscle usually responds well to deep pressure and once it "releases" the sciatica is usually resolved immediately. This muscle is very commonly the cause of sciatic compression, so it is worth giving this a try.
Now, again, that is just one simple cause of sciatica. Other cause can be far more complex and require knowledgable medical care. DO NOT use this type of technique near your spine even if you think that the pain originates there!
If you have/can afford a licensed massage therapist, give him or her a try.