Rajesh links to Brad Feld's wonderful guideline for bootstrapping:

"Pick a domain and go deep: If your answer to "What kind of company are you going to start?" is something like "Well, I have a few different ideas..." stop immediately. You should pick one idea in one domain and go extremely deep on this idea. Optimally, it'd be something you already know a lot about so that you'll be leveraging your personal experience and presumably one of your passions. Hedging your bets by thinking about and playing around with a variety of different ideas is a huge waste of energy - you need all of your focus on the one thing you are going to do. Early in my life as a VC, I was in a meeting where an experienced VC asserted that one of the most important questions for a venture-backed company to answer is "What do you want to be the best in the world at?" I think this question broadly applies to all entrepreneurial endeavors."