Don't let the company get distracted or excited about other things. A common mistake is that companies get excited by their own PR.
You have to be intense. This only comes from the CEO, this only comes from the founders.
Obsess about the quality of the product.
If you compromise in the first five, ten hires it might kill the company.
For most of the early hires you make in a startup, experience doesn't matter very much, and you should go for aptitude.
When lack of structure fails, it fails all at once. What works totally fine from 0-20 employees, is disastrous at 30.