How can I boost my wireless signal on my Airport Extreme?
How can I boost the signal from my Airport Extreme?
Mac OS X (10.7.2)
You don't mention this in your post, but I assume that you want to be able to do this using wireless only and you will not be connecting an Ethernet cable between the AirPorts.
If your AirPort Extreme was purchased in the last few years, you have a dual band device. So, if you want to be able to "extend" or "repeat" both bands using wireless only, you would need another AirPort Extreme.
If you only need to "extend" one band, then an AirPort Express would handle that task nicely.
A good location for an "extending" device is a point that is about half the distance from your main AirPort Extreme to the area that needs more wireless coverage.
Apple's instructions to set this up are here: