apart from doing excellent work it's about using that big thing in the middle of your head called your mouth!
If you clean carpets to a very high standard and then take the money and run you havn't formed a bond with the customer, you have to tell them what you've done why you have done it,
7 things to help you form a bond with your customer and get them beating a path to your door for regular cleaning!
1. tell them that the dirt was trapped in the walkways because you use them more
2. how you can stop this and make your visits less frequent therefore saving them money(obviosly not

)
3. the importance of cleaning
4. the benefits and how they can look after the carpets
5. dealing with future spots and stains
6. show them a half done carpet or the dirty water you have rinsed out of their carpet and how it was harming the carpet fibres
7. (only if you are good at this) when giving them the bill tell them approx how much a carpet like that would cost to replace nowadays and how cheap it is to have them cleaned.
Shaun
PS there are probably more things to add to this list but it's a good start.