1) Does the agent tell you everything you need to know? When you don’t like a certain airline, departure times or dates, or connection, does he shift gears immediately to find alternatives or try to force the issue? If your agent responds well in these situations, stick by him.
2) Don't pick the first agent you find. Talk to the travel pro. Find out how long he or she has been in business. Ask about fees (yes, they charge booking fees, but they're worth it if you get into a pinch).
3) Good travel agents have an edge over almost any other seller of travel. They know what you want. They speak your language. And they're there for you when you run into trouble.