I got told by an NHS anaethisist (sorry for spelling) who also worked privately giving advice on vitamins and minerals and transfusions for B12 and other vitamins.
His advice to me was to have a bath for at least 10 minutes in plain bath water with 2 large cups of epsom salts, he said this was the best way to get a good dose of magnesium directly absorbed through the skin. He said it was important not to use bubble bath etc and not to wash either, just soak as much of my body as possible in the deepest water possible.
I have chronic pains on a constant basis, and it really does help, my aches will really reduce. I bought a 25kg bag of it on the internet, I think it was actually supplied by a farm, as they have uses for the epsom salts too. I use them just once a week, the rest of the time I use bubble bath, and the bag has lasted over a year.