is sometimes not the only question you should ask a travel agent, even though that is the only question which usually comes to mind. Very few of us even use agents to book trip nowadays, but it helps to be really geeky this once and know your flight's seating configuration, if you are really picky about where you sit.
I recently came across a website called seatguru.com and it was a revelation. You choose your airline, the aircraft type, and voila! - you have a seating chart. But it doesn't stop there! You can hover over seats using the mouse and see descriptions of whether there's limited legroom, whether the toilets are nearby, what the seat width is, whether or not the seat even has a window (believe me, there are 'window seats' on certain aircrafts without a physical window, I was in one on a long haul flight once and it wasn't fun!).
You will see a legend on the top of the page for each craft type, which indicates whether the seats are good or poor, and there is also information on whether the seats have their own in-flight entertainment systems or have an outdated central system (an important fact often overlooked when booking cheaper flights).
Of course, you're thinking, how do I know what aircraft type my flight uses? It says AA 55. Is that it? No, that is your flight number, not the aircraft type. When you book your flights online, you should be able to drill down to the level of detail where it mentions a flight type such as Boeing 767 or Airbus 330. That is the flight type. Or just ask your agent about it if you book offline.
Of course, there are no guarantees that you will get your dream seat, but it won't hurt you to try either. Good luck.