Recently I read an article about how many injuries are encountered with removing an avocado pit. This methods works best with a thick skinned, dark avocado.
Using a knife to whack the seed out never seemed like a good idea to me. When an avocado is perfectly ripe, (check by removing the tiny stem cork...if it is brown under the cork, it is ripe) it can be peeled like a tangerine. Start at the stem end, use a knife to cut a small starting point. Then slide your thumb under the skin and peel. When peeled, cut in half and gently coax the pit up to loosen, it comes right out.