Your current method only changes the PATH for the current session. Once you log out, anything you've added to the path will be removed.
The way to modify the path variable permanently is to edit .bash_profile (assuming you are using the default bash shell).
To edit the .bash_profile, change to your home directory (IE: /home/username) and type: vi .bash_profile then arrow down until you find the line that looks similar to this:
Type i to enter 'insert' mode, then add whatever directory you want to the end of it (remember to use the ':' between directory names). Press the escape key to exit 'insert' mode, type :w to write the file and type :q to quit the vi editor.