Explain your decisions. If family members understand the reasoning behind the terms of your will, they may be less likely to contest it. It is a good idea to talk to family members now, and explain why someone is getting left out or getting a reduced share. If you can’t discuss it in person, at least write a letter of explanation for them.
Use a “no-contest” clause. One way of preventing a challenge to your will is to include a no-contest (or in terrorem) clause. This will only work if you are willing to leave something of value to the potentially disgruntled person. A no-contest clause provides that if an heir challenges the will and loses, then he gets nothing. You must leave the heir enough so a challenge is not worth the risk of losing the specified inheritance.
Prove competency. One way of challenging a will is to argue the person was not mentally competent at the time he signed his will. You can try to avoid this by making sure the attorney drafting the will tests you for competency. This could involve seeing a doctor or answering a series of questions.
Remove the appearance of undue influence. Another method of challenging a will is to argue someone exerted undue influence over the deceased person. For example, if you are planning on leaving everything to your daughter who is also your primary caregiver, your other children may argue she took advantage of her position to influence you. To avoid the appearance of undue influence, do not involve her in drafting your will. She probably should not be present when you discuss the will with your attorney.