Basically, add them to ~/.bashrc, and they take effect whenever you login as that user.
To make systemwide changes, do the same thing, but to the /etc/profile file.
So does /etc/bashrc get executed if a system reboots and no one has logged in yet? i.e. if one has a web server on a VPS that may get rebooted from time to time, is it then the /etc/profile that one set required environment variables from?