Timing of Giving

Property may be given away in any manner and amounts desired during a person's lifetime. A Jew must give his property away before death if he does not want to make the inheritance according to Jewish law (such as double portion to a father's first-born son, etc.--consult a rabbi for details). A person should reserve some money to fulfill the Torah commandment of inheritance. Consult a rabbi.
Note In inheritance issues, the first-born son means the first-born son of the father (although pidyon ha'ben refers to the first-born son of the mother).
Note A mother can leave whatever she wants to her children without being required to give twice as much to a son who is the first-born of his father.
American Will
Since a person cannot give away property after his/her death (since he/she does not own it anymore), an American will has no halachic validity. An American will should be written to keep the inheritance out of the hands of the government and lawyers. In order for a secular will to be effective under halacha, an acquisition should be made that starts to take effect beginning at the time of the kinyan and finalize one hour before the person's death. 
Rabbinic guidance is recommended.