I recently made an Instructable showing how I loosen jar lids with a knife. Commenters pointed out that this method would likely put shards of glass in your food and that there was another way to use a knife to loosen a lid.
The result? It works! Mostly. It's also just as easy as the other technique so by knowing both you should be well on your way to being a ninja with the jam jars.
Step 1: Get the Knife in There
Work the tip of a table knife into the gap between the lid and the jar.
Step 2: Twist and Pop
Twist the knife like you're turning a screwdriver. If it works you'll hear a very satisfying POP as the seal breaks. Opening the jar after that is incredibly easy.
I only bought two jars to test because I already had some extra jam for the last test and now I have two more than that. Anyway, one worked fantastically and the other didn't work at all.
So keep both techniques in mind and you'll find no jam jar that can resist you.