Stop popping veins and making constipated faces just to get to those delicious pickles, strawberry jam, or whatever you've got packed away in a stuck jar. Instead do what Instructables user fungus amungus does: use a tennis ball.
This trick may seem cute, but knives are involved. If you've never dissected a tennis ball before, turn away from your screen — it's going to get graphic. But if you're already a cold-blooded tennis ball killer, you won't have any trouble. Just get any knife (although a shorter blade is best — think large Xacto knife) and stick it in the middle of the tennis ball. Then carve around until you've got two halves. That's it! The outside of the tennis ball provides a good grip for your hands, but the rubbery inside also makes it really easy to grip a jar lid. Just place on of the tennis ball halves over your jar, squeeze, and turn. The rubber should help provide a better grip and get that lid off in no time.
Of course you can always run your jar under hot water, loosen the lid with a spoon, hit the lid with a knife, or use rubber bands, but are any of those options as cute? (If you're not sure, the answer is no.)
Tennis Ball Jar Opener [Instructables]