The Wharton Alumni newsletter had a great interview with two Wharton alums who are entrepreneurs (Isakow and Minskoff). They talked about their mistakes and gave some good advice. Here's a link to the whole article -- http://wep.wharton.upenn.edu/gis/article.aspx?gisID=70
Along with the moderator, Bob Chalfin (an instructor), they said the following:
Minkoff suggested that they be flexible. Successful entrepreneurship requires a business plan that allows for lots of wiggle room. "Your business will never play out like it does in your business plan,' he said. "You have to have flexibility in your structure-your money, your timeframe, your markets. You need to plan for bad things happening-stuff will cost more than you thought, key people will leave, and markets will evaporate.'
Isakow agreed, offering up a philosophical take on ups and downs of self-employment. "I read something early in my career that really influenced how I look at my business: 'Failure is never fatal, and success is never final.' There are always going to be bumps in the road.' He added a bit of guidance of his own, saying that, when starting a company or investing in one, you must do rigorous analysis, no matter how promising an idea sounds. "Don't just believe the story,' he added. "I'm the worldwide owner to the rights for solar-powered lawnmowers. You'll notice that you don't see too many of those around. I believed the story.'