One word: DON'T. Ok, there are exceptions. A charity you're involved with, maybe a friend or family. A trade where there is a tangible benefit, not just somebody using the typical lines of either giving you experience and/or being able to add it to your portfolio.
Anything else and the only person benefitting is the one who suckered you into working for free. Nobody is going to hook you up, send you all kinds of business or make sure your name gets out there. The only thing you'll get known for is as somebody who is gullible enough to shoot for free.
Early on, I got tired of people who would talk that "Hook me up and I'll hook you up" kind of crap. The last one got a little of his own back at him. I told him that sounded great, and then explained how it would work. First off, he would be paying for the work I did (The look on his face got interesting). I then told him that for every paying customer he sent me, I'd pay him a percentage of those jobs, until I had paid him back the entire amount he initially paid me. Then I added that for every customer he sent me after that I'd continue to pay him a percentage as a commission of sorts. Of course he didn't age to this. Why? Because he had no intention of sending me anybody. He just wanted it all for free.