Before you even get to interview with potential customer service reps, you'll want to identify the right type of candidate. Here are a few of the most important characteristics to look for:

• They must actually want the job - Find that rare person who actually likes being helpful and talking to people.
• They must have huge levels of empathy - For anyone, responding to support ticket after support ticket can wear you down. You can learn skills, but if you don't come in with a high level of empathy, you'll break quickly.
• They must know how to troubleshoot - They don't have to be technical, but if they don't know how to work through an issue they'll be in trouble (or even better, someone who enjoys solving puzzles) They should lean towards scientific method and root cause analysis. I test this by giving them a fictional company with a generic customer issue ("your company sells tape deck-to-iPod converters, and a customer writes in and says theirs doesn't work") and finding out what questions they'd ask to get more insight on this issue.
• They must have a fire for defending customers - It can be hard to get customer issues and feedback into the development queue, and often requires some serious determination. If your customer service representative is a pushover, this won't ever happen, and your customers will be miserable.
• They must be able to handle the worst in people - Frustrated customers can be incredibly harsh. If your support hire can't handle occasionally being called a bad name, they should go home. The last customer can't affect the way they'll treat the next one.
• They must know how to have fun and laugh at the world - Customer service is intense and unless you can find the little bits of fun in it, you're going to burn out. I often ask people what they'd do if they get a ticket from someone claiming to be Scruffy McGee, a sentient talking dog. If they say that they'd hang up the phone, they're out. If they say they'd offer them a dog treat, they're in.