How do you keep a client once you've done the work?

In other words, once you've set up their website, once you've put in things like Aweber to build email lists for them, and come up with creative uses for QR codes for them, and put up their Google Places pages, etc., why do they need you anymore?

I want to start marketing for service businesses. It is the type of industry that I myself am in, and it is what I know. So many service businesses (cleaners, carpet cleaners, painters, handyman services, etc.) are terrible at their presentation of their businesses, and I have a desire to help. However, many of these people are definitely the "do-it-yourself" crowd, and if I tell them they need a Google Places page, they're likely to just set it up themselves.

So, how do you gain enough control that they actually continue to need you once you have things set up for them? Do you maybe set up your own website with your own contact information so that leads are all funneled through you? Otherwise you've just set up a system for them that will benefit them indefinitely while you will likely not continue to get paid for the benefits reaped by that system.

I'm an over-thinker, so let me know if I'm thinking this through way too much. I just don't want to get paid a small fee once for setting something up that produces them income for years. If selling a person leads, I don't just want to get paid one time for the lead if I am getting them jobs worth $2000 a month in recurring income. A few of those, and they may not need me anymore and I'll have to look for someone else for those leads. Hopefully you get the point.

Thanks for any input.