If you use bash, add a line like this to
PATH is marked for export by default, so you don't need to use
~/.profile exist, bash only reads
~/.bash_profile when it is invoked as an interactive login shell.
~/.profile is also used by other shells that might not understand the same configuration options as bash.
Terminal and iTerm 2 open new shells as login shells by default. When bash is invoked as an interactive login shell, it reads
~/.bash_profile but not
~/.bashrc. The terminal emulators on other platforms often open new shells as non-login shells, so for example bash reads
~/.bashrc but not
~/.bash_profile. OS X users often use
~/.bash_profile as the personal configuration file corresponding to
~/.bashrc on other platforms, but it is also common to source
~/.bash_profile or to tell Terminal or iTerm 2 to open new shells as non-login shells.
You can also change the path more globally in
/etc/launchd.conf, but in many contexts the path will still default to
/usr/bin:/bin:/usr/sbin:/sbin. And I have only really needed to change the path in shells and text editors.