When you don't know, say so. Never make things up and don't speculate about what the answer might be. Find out the answer on the spot, or promise to find out. Do whatever you have to do to find out, no matter who you have to ask, who you have to call. Call the president of the company if you have to, and don't let him off the hook either.
Every customer is special.
Get to know them. Call every customer by name. If you don't know the name, ask for it again and again if necessary, with a self deprecating joke. They may think you have a bad memory, but will never think you don't think they are special. Never let a customer walk, for any reason.