The first lesson we learned was that all travel agents are not the same. Some are more honest than others. The second lesson we learned was to always come right out and ask if the time we want to go to a given destination coincides with say...a hurricane season.
Later on, we learned that doing some research on our own will save some time and effort when we're actually consulting with our travel agent. We like to have a good idea of where we want to go and what, if any, conflicts could bring some unpleasantness to our vacation.
Another great thing to know is if certain seasons or occasions would make going to a destination more or less favorable. For instance, if you hate crazy crowds of half-naked coeds, going to New Orleans during Mardi Gras would probably leave a bad taste in your mouth. However, if you like crowds of inebriated, scantily clad ladies, you'll know when to pencil Louisiana into your calendar.
If you want to know when the best times to travel to various destinations are, a great place to start is Joseph Rosendo's Where to Go When. His book outlines the greatest vacation spots for each season on a month by month basis. You'll always be “in the know” about the best things to do and see—and when it's safest to do and seem them.