Well business is slowing down a little for some reason, and I would like to come up with some specials for the holidays or just the season in general. What are some of the specials everyone has? I thinking like a certain amount off the total price if you have X amount of windows or 10% off total or something. I want to figure out what would be the best and most effective special would be to possibly get more business.

can you not get customers to have a Bi monthly cleaning schedule or quarterly
lets say you charge a £1000 dollars for a yearly clean, i appreciate i am not use to the USA style of window cleaning, Divide it by 6, £166 and clean the outside one month, 2 months later, the inside, 2 months later, the screens? 2 months later the outside

just a thought

Try to include some coupons while in the special so that they'd keep coming back. I think that's the best way to make your business a bit better.

I agree on the coupons, they are pretty great and can get the customers in easily.
You can also try many other forms of advertising, pretty sure there are some experts around here

You should always have some kind of specials even if you're pretty much lying about it, if you're price is 20$ say that it is 25$ and say you offer a special 20% off the price. It makes customers come that's for sure