Take the dog to the nearest vet or shelter to see if they are microchipped. If the dog is chipped, the vet will be able to give you information how to contact the owner. Sometimes the the chip has not been enrolled but the micro chip company should be able to tell you which vet or shelter the dog came from.
If possible, take the dog to the shelter so that they can take a photo of the dog and list it on their website. If you are unable to take the dog, call your local shelters to report the dog found and make sure to give them as much information as possible, including a microchip ID if you have it. Ask for the ID # or a confirmation # for the dog you report.
Post information on Craigslist: Breed, markings, location where found. Leave some detail out so the owner can identify the dog and NEVER turn the dog over without some form of proof the dog is theirs (micro chip number, photos, veterinary records, etc.) Not everyone has as good of intentions as you do.
If you have a Neighborhood Newsletter or Facebook page, post the information there.
Talk to you neighbors and post a flyers in your neighborhood and surrounding areas
Contact Lost Dogs of Texas and they will create a poster/flyer for you for free.
If possible, wait a few days before taking the dog to the shelter. After 3 or 4 days if you have not heard from anyone and you can not keep the dog, take the dog to a shelter. It may be that the owner has been looking for the dog there.
If you decide to keep the dog, make sure you have exhausted the above methods.