As a general rule we don't do one-offs. We have the time, but I'd rather fill that time with regular work than have an empty spot again the next time round.
We have a standard form we fill in every time we get an enquiry, it lists Name, Address, Post code (helpful with country addresses), Tel no., - the next question is: Do you need a regular service or a one off clean?
By giving them what sounds like a standard choice, you take them off guard - they're much less likely to lie if they think a one-off is an option. If they say "yes", then the next bit is easy - "I'm sorry, one offs start at £50 as they're very awkward for us to fit into our regular schedule."
It's too late then for them to change their minds, they've shown their hand.
Even the ones who say they want a regular job are reminded when I give the quotation that the price is based on them agreeing to every 4 weeks (or whatever)
We still get the odd one who misleads us, but in the great scheme of things we ignore it and move on - our business grows at the rate of about £100/week, so why worry?
Incidentally, we never charge extra for first cleans. I use it as a sales reinforcing tactic: "This price is based on our regular monthly service, we don't charge you any more for the first clean, but only because we expect an ongoing regular arrangement."
It helps to create a "loyalty" situation, and if a client is going to contribute £300 a year to our turnover, for several years to come, then I'm quite prepared to invest the extra time involved in a first clean to achieve this.
Cheers,
Ian