Good advice. Beyond that, I recommend surrounding yourself with critical, intelligent, trustworthy people. I bounce all my ideas off a core group of friends, and generally don't bother with anything until they all give the green light. Besides that, it's just ROI/risk evaluation. Unless you're already a millionaire, it is best to focus on lowest possible investment for the highest likely return. Later it may be worth taking bigger risks.

Once you have started a project, you might feel this idea is not worth doing and you will have some other ideas. But don't change your mind. Finish what you are already building. If you leave this project incomplete and switch to another, you will never have a complete project. Instead note down all the ideas you have. Once you are done with current project, go to that list, see what you want to do next.