How do you get a glass stopper from a glass bottle unstuck?

Spray some WD-40 around the stopper. Then take and hold the bottle under the hot water. The hot water will heat the air in the bottle and force the stopper out.

Pour olive oil in to the area between the stopper and the neck of the bottle. Work it slowly but firmly and eventually once the oil has worked between both the stopper should be much easier to remove. Clean the bottle and the stopper meticulously, to remove any residue of whatever was in the bottle, and also check that the surface of the stopper has not worn away and lost its "powdery" texture as this sometimes causes the sticking.

Place the bottle in hot water and then place an ice cube on the top of the stopper. The hot water will expand the bottle neck and the cold ice cube will contract the stopper.