This information was taken from trucalling-fans.com, but if the website doesn't work for whatever reason or if one has a slow dialup network, then I have the addresses and postcard information here.
CEO, Warner Bros. Network
4000 Warner Blvd., Building 34R
Burbank, CA 91522
You can also post on the WB's Forum
Postcard for WB
11800 Wilshire Blvd.
Los Angeles, CA 90025
Postcard for UPN
It seems that the network is trying to persuade the WB, so my best bet is go for that. But since they're being major fuckheads after cancelling Angel (and now realizing their big mistake because they're asking for multiple Angel tv movies, which seems kinda pointless), I really don't know if they're a reliable source to be asking to pick up Tru. But it's worth a shot.
Print the postcards on card stock, or paste printer paper onto cardboard. Write on the back your age, and gender, and that you want to see Tru Calling Season 2! (And also a strong dedicated reason why you enjoy the show, and why it should still be aired).
Also if you want to fully dedicate yourself to saving the show from permanent cancellation by any network, supporttrucalling.com is the place to go. Support and let your voice be heard. Even if you've never seen the series, just take a look into it. It did sort of start off slowly in its first season, but the season finale ended in a bang and left us with a cliffhanger that we've been anticipating to find out more.
Support, promote, protest, do what you will. Just let your voice be heard.