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Save until it hurts was the first thing Mike Alfred, a co-founder of BrightScope, said about retirement savings, and that's part of his best financial advice to give others as well. "Live below your means," he said, "which sounds very simple in theory but it much more difficult in practice." His brother Ryan, the other BrightScope co-founder, suggests keeping your investments at arms-length so you're not tempted to overanalyze them.

Mike said his biggest mistake was buying in to the dot-com bubble, while Ryan talks about how they funded a large part of their business on their own credit cards.

Read next: The Co-founders of BrightScope Share The Painful Secret to Retirement Success