I don't know the right way to prepare octopus, but I knew that it was tough, so when I randomly got a whole one one that the fish department at the grocery store happened to have, I threw it into the slow cooker.

I really liked it. Very tender. Perhaps my biggest "let's just try this and see what happens" success in food preparation.

Since then, I found a Japanese grocery store that regularly carries octopus (not whole, just chunks of large tentacle). It's not all that cheap, but I do occasionally go down, grab some tentacle chunks and slow-cook them. I really like octopus in at least that form.

OP is a little classier than I am. :-)