Jamie,
You could say to the client , " while i am here and have all my kit with me and set up, is there anything else you would like me to clean today, or just "freshen up " for you ?
It works sometimes , and at least you can say to yourself that you asked and tried and get extra work.
If you didn't get extra work try to work out why, were they skint ?, it was obvious other carpets didn't need cleaning, how did you ask the the question, at what point did you ask the question, etc etc
Once you start to do this you can become much more accomplished in asking for more work and getting it !