When Beth Lewis bought Computer Coach in October 1998, her thorough research of the computer-training company led her to believe it pulled in a steady stream of year-round sales.

Then Lewis, 45, got a rude shock: Business at the Boca Raton, Fla., company was seasonal. In fact, sales dropped 50 percent in the summer of 1999. "On a slim-margin business, that means red ink is flowing," she admitted. "I knew there would be some slowdown in the summer, but I didn't think it would drop like a rock."

So Lewis pulled out her business plan and decided to launch a set of computer classes and related marketing initiatives she had planned for the fourth quarter. By moving up programs on the drawing board, Lewis didn't have time for anguish over lackluster sales: She already had a plan of attack.

"I didn't have to scramble and figure out what I wanted to do next," said Lewis, who wrote her current plan to get a Small Business Administration loan. "I just moved it up by nine months."

When they least expect it, businesses can be thrown off course by market downturns -- or even unexpected good fortune. While some flounder, others are better equipped to deal with sudden change. The key: creating a business or strategic plan, so companies can track their growth and plan contingencies.

"The most important thing people can do on a monthly basis is put pen to paper on how they're going to get customers in the door," said Martin Nebojsic, president of Nebojsic & Associates, a Fort Lauderdale accounting firm and small-business consultant. He compared relying on a plan with becoming addicted to a workout schedule at a gym: "I want to see withdrawal symptoms if they don't do it."

Lewis, who spent 22 years as a publishing industry executive, relies heavily on her three-page plan. After all, she's constantly writing and revising budgets and planning marketing programs and new products. "It gave me the road map I needed, a sense of confidence that we've thought about where we're headed, and we're not just reacting," Lewis said.