changing an environment variable like PATH forever

make the setting persistent:

add this line: export PATH=/path/to/dir in your ~/.bashrc if using bash, or ~/.zshrc for zsh:

$ vim ~/.bashrc

export PATH=$PATH:/path/to/dir

:wq

or:

$ echo "export PATH=$PATH:/path/to/dir" >> ~/.bashrc

LATER EDIT!

Each time you execute a bash (non-login) shell instance, it reads and executes the .bashrc file in your home directory. Login shells, on the other hand, do that for .profile file, located in your home directory. You can find the difference between login and non-login shells by reading the bash manual.

In your case, open your ~/.bashrc and set there the variable that you want. For example:

PATH="/some/new/path:$PATH"

Save it, and reload it:

$ source ~/.bashrc