Hi fellow hosts!
Well, after seeing my overall star ratings start to slip recently due to the ridiculous changes to the review system, my ratings have just gone back up from near to the point of losing Superhost status (4.8* overall) to 4.9*! I think my strategy may be working and just want to share it with you (I have done so before on other posts but that was the 'trial' stage - I am now seeing results!). Thanks to those other hosts who have shared similar ideas to this.
Firstly, I have put the following blurb on the first page of my guest manual - it is just underneath the house guidelines and Wifi password and I ask all my guests to spend a moment reading this page when they arrive (I'm hoping that the Wifi password prompts them to actually do so!)

Secondly, I now send the following message to all my guests once I have reviewed them:
Dear xxx, thank you so much for booking with me - you were wonderful guests and I hope you enjoyed your stay as much as I enjoyed hosting you. I have left you a 5-star review and hope you can find the time to write a review for me so I can continue to offer my home as a budget-friendly, safe and comfortable place for other guests. Thanks again - it was really lovely meeting you both! Best wishes, Kath ๐
This message hopefully prompts them to leave a review (in order to keep up the minimum 50% review rate) as well as letting them know I have given them 5 stars in the hope they reciprocate. (Of course, this is only for guests who I have actually left a 5-star review for - I won't be sending this message to guests who have received less than 5 stars). While it does feel a bit like emotional blackmail, I think we do need to educate our guests as most of them don't realise that the overall star rating is based on the 'expectations' question and many guests will just say 'as expected', which only gives you 3 stars overall (even if they have given you 5 stars for everything else).
Please feel free to do something similar with your guests in order to keep up your ratings. Happy hosting 2019!