You need to add it to your ~/.profile or ~/.bashrc file. 

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

Depending on what you’re doing, you also may want to symlink to binaries:

cd /usr/bin
sudo ln -s /path/to/binary binary-name

Note that this will not automatically update your path for the remainder of the session. To do this, you should run:

source ~/.profile 
source ~/.bashrc