I simply run campaigns on my facebook business page. I did one recently, where if my customers recommend me and i get the job. They get 50% off their next clean.
This works well.
I do know somebody however who gives the first 2 cleans for free and someone who offers their fourth clean for free.
Tho he also keeps his prices low to try and keep competition out. Lol im not a fan of his methods.
Or do the UK window cleaning companies option wher the first clean is free to win you over! Then spend the rest of the months working for nothing and gaining no repeat work 😂