I have the same two tools that Holly shows above, except that mine are green, hers are blue. I've also found that running hot water on the jar lid helps. And breaking the vacuum with the pointy end of a "church key" tool (usually used to punch holes in the top of cans) almost always does the trick.
I have a bigger problem: how do you get the items off a shelf in the market when they put the best, most popular items way up high on the top shelf, and you are 5'4 or under? My one method is to go running around the store looking for a tall guy who works there (or more rarely, I find a very tall woman) and asking them for help.