Sorry, entrepreneurs. you're not going to live forever. And neither will your companies if you fail to prepare for the estate taxes your heirs will face after your death.

"When you die, completely foreseeable tax forces will be set in motion, and they will inevitably destroy your enterprise if you haven't prepared a financial structure that can handle them adequately," warns Neil Alexander, a Los Angeles financial adviser who is a member of M Financial Group, a national network that serves business owners.

The crucial ingredient in any estate plan is insurance tied to the business owner's death, the death of both the owner and his or her spouse, or other events related to the company's particular situation. Alexander says, "A well-planned insurance policy guarantees that heirs won't have to sell off the company in order to pay the estate taxes."

It takes in-depth analysis to decide which type of policy -- and how much coverage -- fits the needs of the owner of a privately held company. "For example, your company may already hold a key-man policy on your life. That might make perfect sense from a corporate-cash-flow point of view," says Alexander. "But if that policy is worth $5 million, your company's value will automatically be $5 million higher after your death. Your heirs must be prepared to pay $2.5 million or more in additional estate taxes."

The bottom line: beware of advisers who offer cookie-cutter suggestions about estate-planning insurance strategies. "Your estate planner and an insurance expert need to evaluate your total corporate and personal picture," advises Alexander.