Since timing is everything in the kitchen, you'll want to open a jar quickly and get back to your saute' or souffle'. Try these tricks.
If the jar is new, use a triangular-tipped bottle opener to apply pressure and break the vacuum seal. Do this by placing the triangular tip underneath the lid and pulling the lid away from the jar until you pop the seal.
If this doesn't work and you suspect that food is stuck in the lid, tap the lid with a wooden spoon to knock away the food.
Wrap the lid in a dish towel to give your hands traction as you twist. Or put rubber bands around the lid to get a good grip. The rubber bands that hold together bunches of vegetables are ideal.
If the lid still won't budge, run the lid (not the jar) under hot water for a minute. This will cause the metal lid to expand so it comes off easily.