One small piece of advice to add into this wonderful thread. Always get paid. Always charge something. The payment can be in the form of barter, it can be a dollar it could be lunch it could be something, but if someone pays you for your services, you can use them as a client reference and not just a friend you have done some stuff for. Hell, right now to build my character illustration portfolio I am doing 5 for $.01 for friends. Building up volume and quality that way.
The other point that I am not sure has been beat on as much as it should is that you should only put YOUR ABSOLUTE BEST WORK IN YOUR PORTFOLIO. If you see any mistakes or issues with the map, take it out of your portfolio and fix it before you show nit to anyone else.