I try to write something catchy and funny on my discs with my digits, hoping that if I get a laugh they'll be more likely to return it. Some in my bag are...
"Free Karma Points -Ask For TJ"
"Dont Be Wack-Give this Back!!" (personal favorite)
"Are you my homie? Then show me!"
It's worked a couple of times...but I've also had a couple non-calls that were lost forever. Disc Nation is based here in Austin, they have a lost and found policy at their retail store where if you bring in somebody's disc, they will hold it there until they can come pick it up, and you get a few bucks off something in the store (as well as some good karma). It's brilliant b/c it always gives me an excuse to buy a new disc, as well as provides motivation for those with less integrity to bring a lost disc back.