This was an interesting thing one of the Apple techs suggested to me. When deciding where the optimal place is to put Airport Express devises, turn on the wifi on your iPhone and use that to determine weak areas. It's quicker than my method which was walking around with a laptop until it fell off wifi and then turning wifi off and on again, which I always had to do.
There are a number of decent WiFi analysis apps available for iOS devices in the App Store. ranging in price from free to as much as $4.99 for some "bundles" of multiple analysis apps.
