Small-business owners often are so busy working that they don't take time to plan for their retirements, reported The Wall Street Journal.

But that is clearly a mistake, financial advisers say.

In turn, advisers are helping these entrepreneurs more accurately value their businesses as well as their personal expenses, and figure out how to spend their time once they retire.

Financial planner Tom Orecchio says one of the biggest mistakes small-business owners make is relying too heavily on the sale of their practices to fund their retirements. The Westwood, N.J., planner with Modera Wealth Management says this is a problem largely because they frequently overestimate the value of their business.

"Monetizing a small business is as much an art as it is math. Certain circumstances can have a big impact on value and salability," he says.

For example, Mr. Orecchio had a client who valued his engineering firm at $1 million. Yet the highest price he was offered for the firm was only $725,000--more than a 25% discount off his valuation.

The firm was his main asset along with IRAs and brokerage accounts worth $700,000. So rather than having $1.7 million, he'd likely have about $1.4 million for retirement.

"That may not sound like a big difference but using a 4% distribution rate, it's a difference of $1,000 a month in income in retirement," he says.

Mr. Orecchio says the client was offered less because too much of the business's growth was solely dependent on him. The planner had warned him repeatedly that he needed to change his business development practices and groom successors so that he could eventually fetch a higher price.

Factors such as an older client base, a business too dependent on several large clients and slow-to-no growth can cause a firm's value to be discounted, he says.

"Position the firm for sale early on so that when it's time to sell, you've addressed the necessary things to make it more attractive in the marketplace," Mr. Orecchio says.

When preparing for retirement, small-business owners often make the mistake of underestimating their personal expenses, says Bert Whitehead, president of Cambridge Connection in Franklin, Mich.

Mr. Whitehead worked with the owner of a manufacturing company who expected to spend $15,000 a month on personal expenses in retirement.

He overlooked the fact that his business paid for many of his expenses such as his car, club dues, computers and many travel expenses to the tune of $100,000 a year.

"Entrepreneurs invariably underestimate how much their living costs are to maintain their standard of living," he says.

When the client does retire, Mr. Whitehead says he should allocate a greater share of his budget to fund those personal expenses.

Small-business owners often don't take sufficient advantage of certain products to save for retirement, says Ann Minnium, a Scotch Plains, N.J., financial planner at Concierge Financial Planning.

Ms. Minnium worked with a 62-year old architect who saved as much as he could every year into his SEP IRA but still needed to save more for retirement.

She suggested he open an individual 401(k) and fund that instead of his SEP IRA. An individual 401(k) is easy to set up and would enable him to save more on a pretax basis, she says.

Namely, when the client made $50,000 in 2013, he was able to make a $32,293 contribution to his individual 401(k). If he had the SEP IRA alone, he would have only been able to contribute $9,293, she says.

Since so many small-business owners' identities are wrapped up in being a business owner, Eddie Kramer helps them figure out what to do with their free time once they retire.

The Columbia, S.C., planner with Abacus Planning Group asked the owner of a construction company who was looking to retire in about five years questions such as, "As a child, what did you want to be when you grew up?" and "If you found out you had five to 10 years to live, what would you change in your life?"

Through these questions, he discovered the client loved art. As a result, the client decided to enroll in art classes at a local university as he gradually gave control of his business to some key employees.

"Business owners need something to transition to," Mr. Kramer says.