I got this from an old copy of the cookbook from the Pismo Beach Chamber of Commerce. Apparently they no longer print it.
Probably because there aren't any Pismo Clams left.
This article from the LA Times tells the story: Pismo Beach was Once the Clam Capitol of the World...

To open, use a short, stiff-bladed knife. 
Set clam upright with hinge side in left hand.
Insert blade about one inch up from the bottom, and cut in and down. (You are cutting through one of the muscles that holds the clam closed.)
Turn clam over and repeat process from other end.
Pull shell apart and wash thoroughly.
Lay on side and cut mean from shell on both sides.
Pull off the trimmings.
The dark portion is the liver, which is good to eat, as is all of the clam.
Update: Only the white meat of the clam and the muscles that hold the clam shut are safe to eat. Any digestive portions and dark meat should be discarded.
Also, the muscles that you cut that held the clam shut are good to eat with cocktail sauce.

Photo from Dana Point Fish Company