I took up the challenge of working at the local animal rehoming centre where I live, around 2 years ago. Best move ever!
Grab a squeak from an old toy, these work great for grabbing attention.
Agree with Sirquack, stay away from costumes, as they are more degrading or unnatural for a pet.
Hard faced looking dogs, (Staffs in the uk) or bull breads: get them to have a good walk around the facility or play in the gardens. This will make them pant more, which makes them appear to be smiling, plus you can grab some action shots too.
If you have time, spend 5 minutes with the dog, with treats to help it relax a little with you. Obviously speak with the rehoming assistants before hand.
Final one, when a dog is sat or posed, Pant at the dog. Most dogs will cock their head to one side with a quizzical look on its face making a great photo!
Usually use 70-200mm, 24-70 or the trusty 50mm.
We usually produce a 2 min movie for each dog, on walk, at play etc. Great if the pound / rehoming centre has a web page with the facility.