I have just remembered something cheap and simple, yet indispensable for me, and it's
this.
Over the years I have bought and tried all sorts of gadgets for opening jars, but this is far and away the best and I buy them for my friends and family. All they do is increase the friction between your hand and the jar top (or bottle top). As I have got older my grip has got much worse, so I use these things a great deal more. They don't fail me.