is there a trick to grow avocado from seed?

Patience is most of it. The toothpick method works: three toothpicks into the seed, rest it on a glass with the broad end down and the bottom half in water. Somewhere warm, change the water every few days. It can take a couple of months to split and root, people give up just before it does. Just know it'll be a nice houseplant for years before it ever fruits, if it does, and the fruit may not be like the one you ate.