Check CL and other online sites, and post your own LOST ads online.
I have managed to return several lost dogs. One I located the owners, but we reached mutual agreement that I should keep the dog, as they were not up to keeping her (she is a great dog, BTW)
The dogs I got back fastest and easiest were micro-chipped…well actually the fastest had a tag with the owners up-to-date phone numbers, but collars can be slipped, tags lost or illegible. In this case, a pair of dogs had escaped and only one still had the collar, fortunately I was able to lure them in. The collar is a good, cheap first line of defense, but the chip is a great backup. The downside of the chip is that people have to think of having the dog scanned, and then schlep them to a place that is open and can scan them, so even with a sharp finder it can be the next day before they get you. Be sure to update your info for the chip if you move or change phone numbers.
For dogs with no collar or chip, I post Craig’s list ads, and also the local newspaper, which runs found-pet ads for free. No actual owner has ever contacted me through these, but I get many inquiries from desperate owners checking anything close. No, I wouldn’t mistake a Boston Terrier for a Yorkie, honest! There is also PetHarbor.com and the national lost pet registry online. Be sure to look for the “Pets found by individuals” link on the PetHarbor site. By default they only show you data from shelters and pounds.
Facebook? Not sure where I’d put a posting where anyone at all could find it, but may look into that next time I pick up a stray.
Putting up paper posters around where I found the dog has worked best. People want to physically look for the dog rather than use more efficient media. If someone wants to keep the dog, you are SOL anyway, so assume that whoever found the dog is now trying to find you. One I got back because I was putting up posters and saw a man who looked like he was looking for a lost dog. He had just walked past a poster without seeing it…so look for posters while looking for your dog!
So chip your dogs people, and train them to trust strangers. I am only able to rescue about 1/4 to 1/3 of the dogs I see loose, most of them are too skittish.