$500 awarded to the person with the best idea on tracking print ads, seem like a pretty interesting problem: http://www.ideaconnection.com/contests/tracking-advertising-00008.html Some more here: http://www.ideaconnection.com/contests/
It depends on what you are selling with the print ads. Traditional methods have always been to print a code on the print ad, and ask the customer to enter that code upon registration. You could also give each print as a unique number, and provide 'instant prices' where they go online and register, enter the code on their print ad, and see if they've won something.