I think first and foremost is bringing yourself into a position that gets you enough runway (time and money) to be able to take things seriously — even if that sounds like the opposite of a side project. For me it was skipping university and having kids in favour of starting a web agency in the dotcom days. Also, I don't believe in working on something you love — you just need to hate it a little less than the other ideas and be positive that someone will find it useful. Like I mentioned below, luck and timing play an important role and these days I'd say you will also need one unfair advantage (contacts, cash, distribution, etc) to increase your chances.