If a client wants their site to be mobile-friendly (and is actually willing to pay for it), then the best way is to create a separate site designed specifically with mobile users in mind. Little Springs Design discusses this in their article about Mobilizing vs Miniaturizing.
If the client’s on a tight budget then you’ll need to try and make the same site work in all environments by adding an extra stylesheet for mobile browsers. A List Apart has a good breakdown of the different browser capabilities and a method for cajoling them into working in a consistent way. Additionally, Perishable Press has some quick tips that also come in handy.