In order (if unsuccessful try the next on the list and always put a note through)
Knock at teatime
Knock at 8 am Sunday morning if no result put a note through asking her to send a cheque. Write on it that if you don't receive the cheque within 3 days you will assume she will leave money with one of her neighbours and you'll call either side and explain to them why you are calling to collect the money from them.
I would stop there but "some" might suggest:-
Put a placard in her garden saying " Mrs XXXX - bad payer - stiffs window cleaners for £XX!"
I would never egg her windows, return the dirt to her windows, pour quicksetting cement down her drain "to stop the leak".
