People are living so long these days that you might decide that your 90th birthday would be a good day to draft your will. Bad idea.

Whatever your adult age, it's wise to have a will. Go to a lawyer as soon as you can and write a will, or update the one you have if it's more than three years old.

You probably will accumulate during your lifetime more wealth than you had ever expected to leave to your heirs, thanks to all those 401(k) and IRA funds. That's all the more reason why you would be wise to put in writing just who will, someday, inherit your assets.

To do this, you simply engage a lawyer to draft a will that states your instructions for how your wealth is to be distributed after your death. You sign it (usually in the presence of witnesses) and you put it in a safe but accessible place. Officials of your state will see to it that your intentions are carried out, through the public process called probate.

More than mere wealth may be at stake. Only if you have a properly executed will or trust can you appoint a guardian for your children or make special provision for an aging relative or a handicapped child or a significant other.

And here's another reason to have a valid, unshakable, up-to-date will: No family situation brings on more stress than dividing up Dad's or Mom's estate after a parent dies. One academic study has shown that when no leally binding instructions were left behind, arguments among the heirs were four times more likely to occur.

Here are some do's and don'ts about wills:

Do hire a lawyer to draft your will — and your spouse's. Yes, you can write a will yourself, using one of the many how-to books or the forms published by some states. But if you make just one slip, your will may be worthless.

Only your lawyer knows what your state will consider a valid document. Do get witnesses. Most states do not accept wills or trusts that have not been vouched for by witnesses. Don't ask a beneficiary to be a witness; the will may be legal, but the beneficiary could lose his or her legacy.

Don't put your will in a safe-deposit box. Some states require that a safe-deposit box be sealed when the holder dies, and it takes time to get the will released.

Do use percentages rather than dollar amounts when making bequests. If you don't, much can go wrong. Take the sorry case of a man who left all of his $100,000 estate to his beloved sister, except for $10,000 that he willed to his nephew. But when the man died after a long illness, medical bills had shrunk his estate to only $12,000. The nephew got his promised $10,000, but the unfortunate sister collected only $2,000. The man would have been far wiser to have left his nephew 10% of the estate. In that case, the sister would have collected $10,800.

Do review your will once every three years — and more often if there is major new tax legislation or a significant change in your family status. You may want to change some bequests.

Do revise your will if you move, particularly from a common-law state to a community-property state, or vice-versa. In a community-property state, almost any assets acquired during marriage are jointly owned by both partners — except for gifts and inheritances. In a common-law state, assets are owned by the person who buys them.

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