I think for almost every dog that I've watched, I've sent at least one photo where their dog is snuggled up on my lap on the couch, either sleeping or just relaxing with me. I always like to send photos that show a variety of activities, like going to the park, playing with other dogs, getting a belly rub, etc., but a photo of their dog flopped in my lap always gets a great response. It just really demonstrates the level of care their dog is getting and that their dog is feeling comfortable and happy with me.
The other thing I do is to craft very personalized update messages. It's not just that I tell them "All is well, no need to worry". What I typically do is tell them something specific that their dog did that day, who he/she played with, where we went, or something he/she enjoyed doing. I also keep them appraised on their physical well-being, with comments about their dog's appetite, how they've been sleeping, etc. It shows owners that this isn't just a job for you and you're invested in their pet's well-being.
During the stay I try to take cute or adorable pictures and send them to the client. When the client comes to "pick up", I share cute/funny "moments" their pup(s) did. For Christmas, I have a 3x5 picture printed and ready to hang.
I make necklaces out of beads for my dogs, my clients seem to love the gesture. My repeats come to visit wearing them. its so cute.