In online advertisements we have Google Analytics, SiteCatalyst, and other web analytics solutions to monitor our campaign’s click-through and conversion rates. How do we keep track those things with offline (i.e. non-web) advertisements such as billboards, posters, television, or radio?
At it's simplest...
Give out a custom URL in each campaign, with whatever offer you are making, cookie them to mark campaign source, and redirect to home or sales page as appropriate. From there you track as usual.
If it's appropriate medium you can use QR code to give out the URL.
Yellow pages in Australia are publishing different phone numbers so they can count the hits from the print copy in order to prove they are still relevant ... so far its proving the opposite but thats a different story.
Basically the technique has been to have something which unique identifies the communication medium, different email, phone number, ask for Bob or Jane ... neither exist but you can count ... or failing all that, we are lazy and simply ask :)