If you expect that one of your beneficiaries is not going to be happy with what you left him in your last will and testament, you might consider adding a no-contest clause. Also called "in terrorem" clauses, they make your beneficiaries think twice before contesting or challenging your will. No-contest clauses state that your beneficiary gets nothing -- not even the gift you bequeathed him -- if he contests your will and loses in court.
Check your state's laws regarding no-contest clauses. Not all states recognize them, and among those that do, some are more lax about enforcing them than others. For example, Florida courts won't honor a no-contest clause, so you're probably wasting your time including one. However, Virginia usually upholds them.
Decide whether the clause really will act as a deterrent against will challenges, taking into consideration how much you're going to leave each beneficiary. For example, if you leave someone only $500, he doesn't have much to lose if he contests your will and loses everything. If you've left him $200,000, he may not want to risk that, even if he thinks he should have received more.
Draft your will as you normally would, making your bequests. Include an extra paragraph for your no-contest clause. State that you have "intentionally and with full knowledge" made your bequests the way you have. State explicitly that if any of your beneficiaries -- the people named in your will to inherit your property -- contest it, their inheritance under the will is deemed forfeited. Alternatively, you can state that if any of your named beneficiaries challenge your will, that beneficiary will receive only a nominal amount, such as $5.