Open Terminal, and then type this command, followed by the Return key:
defaults write /Library/Preferences/.GlobalPreferences NSUmask #
defaults write -g NSUmask -int #
The first sets the system default, the second sets the per-user default. The NSUmask may not be honored by software that has not been adapted for OS X. # is the umask (per umask(2)). It may, however, be in decimal instead of octal.