The ones who don't need contacting for access/whatever - just turn up and clean.
If they are in they may pay cash? they might pay chq?
The ones who aren't home, I leave a SAE with a slip which has all the details on it (my address, date cleaned, price, payment details)
Don't bother with putting a stamp on envelope tho it's not worth it
Some guys leave a card with a date on for when they'll call back and collect the money - the choice is yours