Well, to start we could just send signals to nearby stars with planets. Obviously planets within 20 light years are the low hanging fruit. Since we know there's life on Earth, nearby stars might have a greater than average chance of harboring life as well.
There is at least one astronomer who's doing exactly that. I didn't find his name with a quick googling, but I think it was a Russian fellow. Naturally, not everyone agrees on the step from listening to sending focused, targeted bursts.
http://en.wikipedia.org/wiki/List_of_nearest_stars