I have never seen the following technique fail. Use a large (6' or longer) prybar to exert pressure on the joint and arm. This usually means putting a piece of wood on the frame under the car somewhere and pushing downwards on the prybar. It is best to have a hefty friend (don't tell your wife you're using her because of this quality) stand on the prybar. Then hit the outside of the control arm with a short handled sledge. I have never seen this take more than 4 or 5 strikes with a sledge to separate the joint.
Alternatively, it would probably work to use an air hammer and chisel in place of the sledge although I have not tried this myself.
The combination of pressure and shock is FAR more effective than a pickle fork alone. My brother-in-law worked for hours with a pickle fork on his ball joint. When I arrived and showed him this technique, it was apart in two minutes.

I can second the advice to watch out for flying bits of metal. I have a piece of steel in my arm that broke off a hammer I was using in a quarry thirty years ago. Always grind off any mishapen metal off the top of the tool you are striking. In my case, this would not have helped since the piece broke off the face of the hammer.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13