How Do I Execute (sign) a Will?
Wills must be signed in the presence of witnesses and certain formalities must be followed or the will may be invalid. In many states, a will that is formally executed in front of witnesses with all signatures notarized is deemed to be “self-proving” and may be admitted to probate without the testimony of witnesses or other additional proof. Even if a will is ultimately held to be valid in spite of errors in execution, addressing such a challenge may be costly and difficult. A potential challenge is best addressed by executing the will properly in the first instance. A later amendment to a will is called a codicil and must be signed with the same formalities. Be cautious in using a codicil because, if there are ambiguities between its provisions and the prior will it amends, problems can ensue. In some states, the will may refer to a memorandum that distributes certain items of tangible personal property, such as furniture, jewelry, and automobiles, which may be changed from time to time without the formalities of a will. Even if such a memorandum is permitted in your state, proceed with caution. This type of separate document can create potential confusion or challenges if it is inconsistent with the terms of the will or prepared in a haphazard manner.
source http://www.americanbar.org/groups/real_property_trust_estate/resources/estate_planning/an_introduction_to_wills.html