Use a wide, flat wooden spatula or similarly thin, blunt tool and work it between the abalone and its shell. The abalone attaches to its shell with a solid round muscle at the bottom, everything else is just clinging to the shell. Work open a section between the abalone and the shell, and then firmly but gently work the spatula around and along the shell until the abalone detaches. Push against the shell with the tool rather than the abalone for easiest release and to keep the abalone whole.
Note: As you can see, I like to do this in a sink for the easiest of clean-ups.