The link that @corleone posted shows how the $PATH environment variable gets interpreted.
The reason why it does not persist when you declare it in the terminal is because that is a temporary process that will eventually exit. The variable has to get set from a file that is sourced when the shell is launched, which will normally be one of your dotfiles.
Unlike many operating systems, OS X does not automatically include a .profile or .bashrc in users' home directories, but instead defaults to .bash_profile, which will be evaluated last if you do create a .profile or .bashrc (which will also be evaluated).
If you want to update your $PATH to include specific directories, the best place to put it (on OS X) would be in your .bash_profile, which will take precedence over any other locations that set that particular variable. If you don't want to overwrite anything, but just want to add something, in your .bash_profile, you could include a line like this: