If you find yourself in a big company, you can still be "intrapreneurial" — a term that refers to entrepreneurial activities in an otherwise non-entrepreneurial environment. Here are 10 Commandments of the Intrapreneur:
1. Come to work each day willing to be fired.
2. Circumvent any orders aimed at stopping your dream.
3. Do any job needed to make your project work, regardless of your job description. (BC: Or, as Eric Reis puts it: "In any situation it is your responsibility, using your best judgment, to do what you think is in the best interests of the company. That's it. Everything else [in your job description] is only marketing.")
4. Find people to help you.
5. Follow your intuition about the people you choose, and work only with the best.
6. Work underground as long as you can – publicity triggers the corporate immune mechanism.
7. Never bet on a race unless you are running in it.
8. Remember it is easier to ask for forgiveness than for permission.
9. Be true to your goals, but be realistic about the ways to achieve them.
10. Honor your sponsors.
A gentler version is here, both I think are attributed to Gifford Pinchot.