My guests, upon check out, leave the keys in the apartment. After I come to the apartment, if it is reasonably clean, I send them a message something like "Thank you for leaveing the apartment so clean. I really appreciate it very much!" They respond to the message almost immediately. Something like..."we always leave apartment clean"...."it was clean so we left it the same way"...but point is that guests loves to see that you notice it and they feel better. And do leave a better review! Try it and see:).
And also be sure that during the stay you send them a message something like "Is everything okay? You need something?" They will probably answer something like "Everything is OK! Thanks for asking." But they will apreciate your care and will see you as careful and easily reachable host whom they can turn to at any time if needed.
Here is my third tip!