In general, I consider customer service to be the heart of library service, we help patrons to the best of our abilities. Period. I am also not naive. I have been around long enough to understand that beast of all library customers--the Time Sucker. Time Suckers are often the most needy, helpless and annoying of library patrons. In other situations, they are mean and demanding. They suck time and energy from all staff (and probably everyone else they encounter). Time Suckers are in every library in the world. Experienced librarians are wary and rookies never see them coming.
The bad news is that you can never protect yourself fully from a Time Sucker. You must accept this as just one of the facts of life in library service. The real problem of Time Sucking patrons is that they can cause us to retreat to self protective measures (read: policies) that inhibit good customer service. How do you at least minimize the negative effects from Time Suckers effectively but also keep yourself open to providing good customer service opportunities?
First, forewarned is forearmed. Be proactive with your co-workers about potential customer frustrations. Share with your co-workers about possible Time Suckers. Communicate about regular customers that have "issues". Tag team a problem whenever possible. Remember, you do not get to "excuse" yourself because a customer is difficult.
Next, do a thorough reference interview (good advice no matter what). Lots of Time Suckers have unrealistic expectations about what technology or research can produce. Ask for context and don't be afraid to say that you are concerned about the scope. Ask about time constraints. Ask about the purpose of the research or question. Reiterate that you are a guide and helper in the process. This does not mean you can't share an opinion or discuss options. Suggest a specific appointment time with an agenda so you can control time allocation. Librarians can and should be sounding boards but you also want to communicate the value of your time and the customer's.
Finally, understand that often Time Suckers can be lonely or have psychological problems, be under tremendous stress, annoying opportunists or completely lazy. It is easy (and normal!) to get frustrated. If this starts to happen, break from the action using any excuse you can manage. (restroom, even if you have to feign illness, answer a phone, consult with another staff member). By disengaging yourself from the customer for even a moment, you can collect your thoughts, slow down and rethink your approach and ask for help. Feeling the frustration as the librarian is not unusual or should it be unexpected. Losing control of your emotions or letting your frustrations fly in the face of a customer or in the general public is more damaging to the library and to your career than you can imagine.
Again, you can never avoid this kind of customer completely, but you can arm yourself with humor and acceptance of what library service is all about.
Go forth and serve the public (even the annoying people)!