koan - 11:05 pm on May 2, 2011 (gmt 0)
I think a lot of it is luck with finding the right niche at the right time with the right competition level. After that it's talent, knowledge and hard work. Knowing when to quit a niche or persevere can be difficult. Make sure it can either become very popular in terms of traffic or profitable in terms of commercial interest. Getting those two together is the holy grail.