I've been very fortunate to have a nearly 100% review rate since I started. I consider the bare minimum for a stay to be sending multiple pictures each day with a detailed update about how the dog's day went and what they did. I usually try to have both pictures outdoors and indoors, and I try to do something fun with the dog each day and get pictures of them having a blast - hiking, visiting various landmarks (the river down the road, the stone wall up the hill, etc), playing with us or with our dogs, etc.
Then...I sneak a doggie gift bag into my customer's things when they pick up their dog. Then they find it when they get home. I include a little report card with a clip art star that says "Don't forget to leave a review!" and I circle the star. I also have a few other things in the bag - always a hand-written thank you note (doggie themed blank thank you cards) and a couple dog-themed gifts like a paw-shaped magnet, a wet dog food can lid, a ball toy, a roll of poo bags, etc. (These are all at the Dollar Tree). I usually print out 4x6" prints of one or two of the best pictures I took during their stay as well. I think when they get home, they are so excited to have a little surprise, and the fact that the stay already went really well, they sit down and write the review right then. Hope this helps!