The "best" location for your base station is a place where you can get an acceptable signal anywhere in your area. This is usually an elevated spot-mine is on a shelf 5 feet off the floor. You use the signal bars from your AirPort software to determine whether or not you're getting a good signal.
Another option could be something like Mac Stumbler, a wireless network tracking program that emulates Net Stumbler. These programs interact with your wireless card to give you a lot more data than just a signal strength, and there's some discussion about how "ethical" their use is. My take is that if you aren't using something to hurt or rob someone (like taking wireless access without permission), then there's no problem.