You could take notebook with you?
I'm not saying put a quote on every card, it's just handy if you can see the backs of neighbours houses whilst cleaning - then I pop a card thru
Also, if you do get someone calling the conversation generally goes "hi, you've put a card thru my door saying you can do my windows for £...." then I just get their address and happy days

If the conversation goes " you've put a card thru my door about window cleaning" I ask them if I put a price on it - if their lying you'll soon know - I can only speak from personal experience but it's worked for me and no one has ever tried it on