Pretty cool question. A detailer should attempt to build an understanding of the client, in order to learn their expectations. They should be able to look over the car with the client, and instead of just trying to jump into a sale on their most efficient and profitable service, they should land the sale that's going to match or exceed the customer's expectations. If you're running your business right, you should know your cash crop services. But at the end of the day, the thing that gets your business flowing is that word of mouth advertisement based on customers that you built a relationship with that realized you weren't just throwing them into a forced sale on something they might not need or might not be able to maintain themselves (if they're the ones doing their own maintenance).
This requires good communication skills, good pre-inspection skills, and being organized to operate at maximum efficiency. Just spend some time before/after doing the work getting to know the customer, and that's something that is going to translate into loyal customers who are going to spread the word about your business and your work. And yes, understand your skillset, and offer work that you're willing to put your name to. If you're doing this for a living, you should take the time the learn the tools and the products and understand the process this work entails, and be willing to explain your services to the customers in a way they can understand.