You have to find a small market in which you can get a monopoly and then quickly expand.
You need conviction in your own beliefs, and the willingness to ignore others naysaying.
For most software startups, this translates to keep growing. For hardware startups, it translates to don't let your ship date slip.
Move fast. Speed is one of your main advantages over large companies.
In YC's case, the number one cause of early death for startups is cofounder blowups.
For most of the early hires you make in a startup, experience doesn't matter very much, and you should go for aptitude.