Apple - How can I add a path to the PATH environment variable?

OSX reads the following files in order when a terminal opens :

/etc/profile
~/.bash_profile
~/.bash_login   
~/.profile     

So place your path addition into one of these. I normally put additions into ~/.bash_profile

echo 'export PATH=/my/path:$PATH' >> ~/.bash_profile should do the trick!

If you used echo 'export PATH=/my/path:$PATH' > ~/.bash_profile, or any such variation, you would be overwriting the contents of your profile!

In addition to the places mentioned by  @RobZolkos, the login shell also looks in /etc/paths and the files inside /etc/paths.d/. One path entry per line in these files.

See /etc/profile and the manual page for path_helper.