Many people know the following about starting an avocado tree from a seed/pit:
Stick toothpicks in the side to hold it up in a jar of water. Wait for it to grow. Plant.
However, did you know there's a trick to getting the tree to grow straight up and really tall really fast? Basically, what you do is keep it somewhere dark until it's ready to plant. Because of the lack of light, it keeps growing up taller, and it grows straight (searching for light), since there's no light to bend toward.
Anyway, we used to start them in my grandma's cellar when I was a child. Later on (still as a child), I started one in a locker in my bedroom, and it grew very nicely; I think it got about four feet tall before I took it out of the jar.
Another advantage of keeping it in the dark is the water likely won't grow algae in it.
Location: SW Idaho, USA
USDA hardiness zone: 6
Elevation: 2,260 feet Profile post