1) Offer a reward.
2) Post fliers on telephone poles/other visible, legal, places.
3) Post and search on the "lost pet" section of Petfinders.com.
4) Put an ad in the newspaper.
5) Notify the pound, animal control, and the SPCA.
6) Ask local kids.
7) KEEP LOOKING. In the same places, over and over.
I tell you this because when I found my foster ig in a tree last year, I looked in the newspaper, on Petfinders, and around the neighborhood for posters. I called the local SPCA and pound. When I saw that the owner hadn't bothered to use any of these resources, I adopted the little green guy out to a new owner.
Also, _don't give up_. It was about August when I found that little guy- according to neighbors, he had been sighted on and off for THREE WEEKS. He was fat and green- and had been just fine, thank you, for three entire weeks in a tree in inner-city Philadelphia. If the owner had persisted in putting up signs and making inquiries, I would have been able to return his iguana to him.
Most importantly, ask any kids you see in your neighborhood, and tell them about the reward. Hand them the flyers. I guarantee you that a 10 year old boy will notice an iguana in a tree weeks sooner than an adult would. This is exactly the type of thing kids that age are dying to find... and keep. Unless you're offering something better, like money! Try not to approach them in a "Hi, I'm a kidnapper" sort of way, though... I remember our grammar school teachers always told us to beware of people looking for "lost puppies"! 
Let us know what happens- it's a terrible feeling to be missing your pet, I know.
-Erin