All of Patrick Rhone’s advice on the subject is good, but this is great:
Seriously, just be up front with people and have a willingness to help them find a solution even if you don’t know it yourself right away. Sometimes, someone just needs a hand to hold to work through a problem. You should be ready to offer both of yours when needed.
Before I took my current position, I ran the service shop for an Apple-Authorized Service Provider. We did a fair amount of consulting, and the most important thing I told people was this: “I don’t know, but I know how to find out.”