The Last Resort

How to get a stuck jar open:
a) use a cloth, grip mat or similar. I used to wrap a rubber band around, but the tupperware mat is more convenient. This simply allows you to use more force on the jar, by increasing the friction so you don't slip.
b) run metal lid under hot water for a minute. It should expand, making it easier to remove.
c) both a. and b.
d) ask handy bloke to open it, enhancing his chances with a, b, and c as required, or any other set of grips from his shed.
e) whack jar lid edge firmly against benchtop (potentially breaking an airlock).
f) poke a hole in the jar lid (definitely breaking any airlock).
g) buy a special jar opening tool with a handle - this adds leverage.
h) see picture.

I got up to f, then skipped on to h. The ingredients are
* 1 recalcitrant jar
* 1 sharp pointed knife
* 1 meat mallet
* 1 spare, clean jar
* 1 clean spoon
* 1 knife sharpener

Method: hold knife point down on jar lid. Whack with meat mallet. Keeping knife in the hole so created, use it to cut a line. Repeat a couple of times, creating a rough N shape. Use the knife to lever back sections of the lid - caution, edges will be sharp. Use the spoon to extract the contents of the jar to the spare jar.

This is not good for your knife. Resharpen afterwards. And do look out for any metal shavings from the edge: they are not good to eat.