Mine is to grow the number of domestic customers steadily and get more from each customer. In other businesses, they are always looking to "maximise the income from each account" so in our world, that means selling more services to each customer. I am offering wc, guttering services, power washing of decking, patios, outside furniture and drain clearing. The power washing is fairly new to me but have done a few and got a couple more to do. Guttering is going OK and yet to do any clearing of drains.
Get some leaflets printed and let all your customers know what you can do for them. I think they would rather deal with someone they already know and trust than a stanger. The additional work can be done on the wet days or weekends if you are pushed for time.
Partner up with another local window cleaner and share the cost of new machinery and then share the work?