Evan Williams, of Blogger and Odeo fame, has posted a list of Ten Rules for Web Startups which is just excellent. Many of the ideas I have tried to express on here before, and indeed, they would translate very well into many kinds of businesses.
Focus on the smallest possible problem you could solve that would potentially be useful. Most companies start out trying to do too many things, which makes life difficult and turns you into a me-too. Focusing on a small niche has so many advantages: With much less work, you can be the best at what you do.