There are two things that I think are very important:
1) Be realistic. Do not try to sell luxury goods to a world that is entering recession! You need to think about what customers want to buy, not what you want to sell

Every time you want to do something, ask yourself how your customers and colleagues will benefit.
2) Be flexible. If your business is failing, don't gamble your family home and your kids' food trying to rescue it! If your industry is suffering, retrain as something else. Try to turn each negative into a positive opportunity.
3) ALWAYS keep your business and personal affairs separate.
4) Do not be too proud to do things the old-fashioned way. We all want to live like kings, but sometimes you can take a bus, stuff envelopes, or sew a button back on an old shirt.
5) Maintain honesty and integrity. People may think you are naive in the short term, but in the long term you will have earned a good reputation.
6) Never allow yourself to be unemployed. Even if you have to do volunteer work! You can use the opportunity to learn new skills, or do something you always wanted to (like write a book).