I would Like to add a bit in what Jeff said:
Your site must have:
# unique design
# SE Friendly (you can't make Google unhappy and expect to make a Living out of your Blog )
# Engrossing content.
# Social networking. especially when you start off with a new site, you do need help from others, like addition to blogrolls of popular blogs. THis surely helps in getting traffic.
# Keep the reader base intact and thinking about expanding it.

These are just basic things and if you want to start off with blog, problogger is also a good way to start OFF. He is particularly very good about blogs. Thanks for the SEO Book link Jeff.