Make sure to visit the pound every day and check all cages.
Our kitty was at the pound and even though we went and searched, they had her in an isolation room. We didn't find her in time to save her.

Ask the shelter if there dogs are kept in an isolation room, medical facility, etc. Sometimes, we've even found dogs in with the cats. (due to overcrowding). My advice is to keep going there and searching and make friends with all the volunteers.

Sadly, make sure to look in the DOA file in case Brownie was hit by a car.

Other good ideas: http://www.petrescue.com/library/find-pet.htm

Good luck!