How would you motivate call center agents in debt collection department?

just imagine that your team is a football team, cheer your team, point out the posittive even when they did not hit the target.
offer them small prizes by making competition for like who will hit the target first with a great quality score or the best average handled time...

by giving them incentives i think 

I would schedule meetings and call reviews on an individual basis, as to address specific difficulties, that specific members of my staff are having.  So as to not embarrass or call them out among their peers.  I would also perform an overview of my staff, to determine what problems that they are all being impacted on similarily,  and then I would schedule meetings and role-playing sessions to assist them in managing their calls better.  Motivation would come through telling them to listen and be patient, and genuinely try to assist them with their difficulties.  The most important thing that I found in my many years of experience, communicating with upset customers, is empathy.  Not just saying the words, but meaning what you say.  Because if you come across like your reading a script, or a company mission statement, they will eat you alive, over the phone.  Empathy & sincerity are the two biggest motivational keys.