A good idea is to find out if they want an inside clean.
Try to schedual this during the bad weather. Always mention xmas and you should pick up more work.
Sell yourself by maybe sugesting that you could clean their facia or gutters + conservatorys as mentioned.
I tend to go for after the leaf fall or April time so their house looks good for the summer.
Look out for manky con roofs and offer your services. I know some window cleaners cant be bothered with them. Just dont pinch their normal work though.
Price by the hour, or mtrs if doing gutters/facia. Never under quote as you will be surprised at how long some jobs take.
Hope this helps.