My hands are now incapable of unscrewing any tops and I can't use the device shown by Chris as you need to have some strength in your hands to squeeze it and turn, which I don't have.
However, I was given a tip a few years ago that actually works for me, but you have to be careful when you do it.
For glass jars as in pickles, honey, jam etc, bang the edge of the lid on a hard surface.
I use the tiled floor for this, and just turn it to bang all around the lid, and you should find it will come of easy after that.
For bottles I use one of these.
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