the only thing i offer is a free clean for every custy they get you, works ok
if you think about if if every custy gets you 1 custy every 6 months then you can double your round, might cost you a bit of money in free cleans, but think long term,
if they have been with you for a long time and want to get loyalty out of them offer a half price gutter clean, or a free gutter clean every 12 months, they are with you, as after the first clean they are easy to look after