There is a very simple technique:
Use a pocket knife or similar with a short sharp blade...slightly smaller than the opening of the bottle.
Push it vertically into the cork, almost all the way through.
Then twist it slowly...until the cork starts to turn.
When it moves, continue twisting and start to pull upwards...slowly!
It always works...impresses the boys and girls too of course.
Swiss army knives are ideal (but then they usually have a cork screw so why bother?)
LESSON - never go anywhere without such a device.
Hard-core freaks would have one of the new Swiss army knives with a built in USB stick anyway!