When I’ve got a jar that just won’t open, I tap the outside rim of the lid with the handle-end of a butter knife. Most of the time, this will break the seal and I’ll be able to open the jar easily.
If it’s jelly or some other sticky kind of thing, I’ll also run the top of jar under hot water for a minute. Sometimes some of the sticky stuff got caught in the threads, and the heat melts it, enabling it to open!
Another thing to try, if you’re having trouble getting a grip on it, is to wear a rubber glove. It works like one of those rubberized jar openers, except it does double duty when you’re cleaning.
Linked to: Works for Me Wednesday
March 17th, 2010 | 
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Another great one is tipping the jar upside down at an angle and giving the bottom a good solid whack with the palm of your hand. It also breaks the seal pretty well a lot of the time.
I’ve whacked the bottom of the jar, too, although I’ve never heard of the other tips. I know this is bad, but if there’s a really tough jar, I use the edge of the can opener to pop the edge and break the seal. Eh, it’s not like it’s really “sealed” once you open it, anyway, so what’s a little crease?