As you’ve probably know already, running the jar lid under hot water can help, but some lids need to be exposed to heat longer for it to actually work. So when all else fails, Sarah Jampel at Food52 recommends you try this trick:
Turn the jar upside down, submerge it in a bowl of hot water for a minute or two, and then use a boosted-traction method to twist it open.
You might have to refresh the water several times with more warm water for it to work – to maintain the heat – but eventually it should budge. With enough patience, no jar can defeat you.