I recently followed the tips in this article to see if they would help at all. Changing the channel and implementing MAC filtering means I can now set my AirPort Extreme at 10% power, and I still get full strength signal everywhere in the flat. I also have fewer incidents where I have to manually rejoin my wireless router.
[robg adds: You might also want to set your AirPort up as a closed network -- it won't be visible to the casual user who happens to be located nearby, though this won't stop dedicated hackers from finding it. Note that you may have some devices that won't work with a closed network (I had an old audio appliance that simply failed to connect if the network was closed).]

